Optimizing the Potential Utility of Blue-Blocking Glasses for Sleep and Circadian Health

Abstract

Purpose

Wearable filters that reduce light in the short wavelength region of the visible spectrum, or “blue-blocking glasses,” are increasingly available and offer an individualized, low-cost tool for enhancing sleep and circadian health. However, their effectiveness depends on filtering properties, application, timing, and implementation. If these factors are disregarded, blue-blocking glasses may be ineffective or lead to counterproductive outcomes.

Methods

We introduce a new metric, melanopic daylight filtering density (mDFD), to quantify a filter's capacity to decrease melanopic input, providing an alternative to ad-hoc measures. We applied mDFD to 26 commercially available glasses, estimating their potential to reduce circadian and related physiological effects of light across common applications in the context of consensus-based metrics and recommendations. We also reviewed intervention studies that used blue-blocking glasses.

Results

Products varied considerably in mDFD, with only those rated at mDFD ≥1 providing sufficient reductions in melanopic input to justify the “blue-blocking” label and associated claims. At least one relevant sleep or circadian-related outcome improved with blue-blocking interventions in the studies reviewed. In addition to filtering strength, appropriate timing and usage are critical to effectiveness.

Conclusions

The efficacy of blue-blocking glasses depends on both spectral filtering and proper usage. The mDFD metric offers a consistent, evidence-based approach for evaluating, selecting, and designing products that reduce photic input for non-visual physiological effects of light.

Translational Relevance

Standardized characterization of blue-blocking glasses using mDFD facilitates reliable product comparisons, evidence-based selection, and rational design of lenses that are optimized for circadian health across a range of applications.

Introduction

Potential Utility and Pitfalls of Blue-Blocking Glasses

With frequent headlines about fatigue-related accidents and the adverse effects of sleep disturbance on human health, a putative solution has been gaining momentum in the form of lenses that filter out light specifically in the short wavelength (blue) region of the visible spectrum. In this context, the purpose of the glasses is to shape the spectral composition of light entering the eyes in order to reduce input to the circadian timing system and minimize sleep disturbance caused by biologically potent light too close to desired bedtimes. Additional claims attached to blue light-filtering lenses render them the new panacea in optometrist offices and online marketplaces, including the promise of preventing macular degeneration, reducing eye strain, and even minimizing the risk of certain hormone-sensitive cancers. The possibility of addressing such issues via special glasses is appealing, at least in part, due to the apparent simplicity and relatively low cost of the solution. In view of the growing availability of “blue blockers” or “blue-blocking glasses”, along with the fact that these glasses are not regulated in the same way as pharmacological treatments or medical devices, their usage warrants some careful attention and consideration.

Reaping the potential benefits of blue-blocking glasses for improved sleep and circadian health is somewhat complicated by a variety of factors that ultimately make a one-size-fits-all solution impossible. Firstly, characteristics other than wavelength (e.g. timing, intensity) also serve to mediate the effects of light.¹ In addition, independent of any light filtering, photic exposure patterns are dynamic, varying according to location and behavior, both within and between individuals (who also vary in their physiological responses to light).² The above issues are further compounded by the mutual incompatibility of the range of applications for which blue-blocking glasses are being marketed, as many of these glasses have the potential to alter circadian rhythms, regardless of the intended use.^3–5 Without consideration of these factors, blue-blocking glasses may be entirely counterproductive. Indeed, a haphazard approach to minimizing blue light may lead to more - rather than less - circadian disruption and/or sleepiness, resulting in increased risk for accidents, errors, and negative health outcomes.⁶

Recent work suggests caution should be taken when evaluating the efficacy of blue-blocking glasses. One literature review has questioned the utility of light-filtering eyewear for sleep and circadian applications,⁷ likely, at least in part, because of the significant differences in the transmittance properties of the myriad brands, tints, and styles that are currently commercially available.^4,5 Collapsing findings across the broad range of filters referred to as “blue blockers” or “blue-blocking glasses” will undoubtedly yield mixed results, because while almost all blue-blocking glasses do filter some amount of short wavelength light,^3–5 many glasses with clear or near-clear lenses, which are marketed as such, only provide rather modest changes that are not physiologically significant for circadian applications.

We assert that the terms “blue blockers” and “blue-blocking glasses” in the context of improving sleep and circadian health should be reserved for lenses that disproportionately filter out short wavelength light to a meaningful extent, and that a fair assessment of efficacy should only include research using those types of lenses. Numerous studies of lighting interventions for supporting sleep and circadian rhythms have in fact found that wearing true blue-blocking glasses in the period immediately preceding desired bedtime contributes to improvements (reviewed below); however, there is a need for a metric upon which to predict and compare the extent to which products could produce biologically-relevant reductions in light exposure. Although some metrics have been previously proposed, these largely have potential drawbacks in so far as they are not grounded upon internationally-agreed measurement standards and/or do not provide a singular quantity upon which to meaningfully judge the performance of each product.^3,5,8 An approach proposed by Spitschan et al.⁴ overcomes those limitations, although the linear nature of the proposed quantity is arguably not ideal (and open to potential misinterpretation), given the logarithmic relationship between light exposure and resulting physiological responses (discussed below).

In this article, we aim to define what constitutes true blue-blocking glasses and provide guidance on how to maximize the potential utility of these products for better supporting sleep and circadian health. To that end, we first review the circadian timing system and other non-visual physiological effects of light. We also detail the various elements that mediate those effects, with photic wavelength being just one of several relevant factors. Next, we briefly describe the consensus-based metric for quantifying the physiological effects of light,⁹ and introduce a revision of the metric put forth by Spitschan et al.⁴ We propose melanopic daylight filtering density (mDFD) be used in the characterization of filters that can affect circadian outcomes, as an alternative to ad-hoc metrics that are still commonly touted. Specifically, mDFD provides a measure of a filter's ability to attenuate direct light-sensing by the intrinsically photosensitive retinal ganglion cells (ipRGCs) that contain melanopsin^10,11 and are primarily responsible for mediating the physiological effects of light in humans.^1,12 Using mDFD, we characterize a variety of commercially-available brands and tints of light-filtering eyewear and provide estimates of their potential to reduce circadian and other related non-visual physiological effects of light, in the context of new metrics and associated recommendations,¹³ and across common lighting scenarios. Generally, the lenses measured in this paper vary considerably in terms of their physical properties and thus, their potential efficacy for specific applications. Additionally, we compare how these measures map on to findings in studies that have employed blue-blocking glasses for altering circadian rhythms and sleep. Finally, we summarize the implications of this combined information and reiterate factors that should be considered when making decisions about the usage of blue-blocking glasses.

Non-Visual Physiological Effects of Light

In recent decades, there has been a growing appreciation for the role of light in a host of non-visual physiological responses, with circadian regulation being among the most critical to human health and wellbeing. The circadian system regulates myriad physiological and behavioral processes, including sleep-wake patterns, hormone fluctuations, and metabolic factors. Those circadian rhythms have an endogenously-generated cycle with a period of approximately 24 hours, even in the absence of temporal cues, with most individuals having a circadian period that is slightly longer than 24 hours.¹⁴ In mammals, the central clock is located within the suprachiasmatic nuclei (SCN) of the hypothalamus and is reset daily to a more precise 24-hour period by environmental signals.^15,16 The most significant cue for synchronization of rhythms (entrainment) is the 24-hour light-dark cycle. Entrainment of circadian rhythms occurs via daily light-induced ‘phase shifts’ that adjust for the difference between the endogenous period of the rhythm and the period of the entraining light cycle. Even just a single brief pulse of light is enough to shift the clock, and both the direction and magnitude of those shifts vary systematically across phases. For example, light early in the biological night elicits a delay in rhythms, whereas light near the end of the biological night advances the system to an earlier time.^15–17 With a human circadian clock that tends to run slightly longer than 24 hours and that shifts to a later time when exposed to evening light, best practices for optimal health are generally to get light exposure early in the day and to keep evening and nighttime environments as dark as possible.^1,12,18

In addition to circadian regulation, there are a variety of other physiological effects of light that are separate from the visual system, including pupillary response, melatonin suppression, acute alerting properties, and mood enhancement. Light for these non-visual physiological responses follows a distinct neural pathway from that of the visual system. Specifically, while light for vision is detected via rod and cone photoreceptors, non-visual responses are primarily mediated by ipRGCs.¹² These ipRGCs contain the photopigment melanopsin and project via the retinohypothalamic tract to the SCN, which regulates circadian and neuroendocrine function. Additionally, ipRGCs project to brain regions beyond the SCN, contributing to functions such as pupil constriction, sleep regulation, and emotional processing.^19–23

The biological potency of light for all of these non-visual responses is affected by photic intensity, with a characteristic dose response to light in healthy young adults.^24–29 The dynamic range of responsiveness to light includes photic intensities within the boundaries of threshold and maximum response. Threshold represents the amount of light required to elicit a response, whereas the maximum response dose represents the intensity at which saturation is achieved (i.e., increasing the intensity of the light will provide no further impact). Dose-response curves for the different physiological functions may overlap, but may not always include the same exact dynamic range, each of which is generally 2-3 orders of magnitude (e.g., Hut et al.³⁰). As an example, if light is being used to acutely increase alertness at night, that same light may simultaneously shift the clock or suppress melatonin, which may not be desirable. Similarly, if light is being manipulated to reduce input to the clock and prevent sleep disturbance close to desired sleep times, such as in the case with blue-blocking glasses, it may also reduce alertness.⁶ However, a particular range of photic doses may, in principle, also alter one particular physiological response without influencing another.³¹ Studies that examine multiple physiological responses in parallel, within individuals, under the same laboratory conditions, and across a range of intensities of the same light source, are necessary to better understand the relationships between dose responses for different physiological effects of light and ultimately, may help to fine-tune interventions to target particular responses.

Spectral characteristics of the light source further influence the intensity needed to elicit a given response. Comparable physiological responses are generally obtained with lower intensities of shorter versus relatively longer wavelength light. Several different action spectra identify a common, reasonably narrow region of peak sensitivity for non-visual, physiological effects of light across species,^{19,25,32–35} and this function maps on to the absorption spectra for the melanopsin photopigment.³⁶ Accordingly, it appears that melanopsin photoreception is the primary origin for most commonly studied non-visual physiological effects of light.¹² Importantly, while the peak sensitivity for all tested non-visual physiological effects of light may be at ∼480 nm, this is not the only wavelength that elicits a response. For example, close to maximal effects of light can still be obtained at 470 and 490 nm, and approximately half a maximal response is still achieved with ∼520 nm. Outside of the laboratory, studies that use white lights with variable spectral quality also show increased non-visual responses to light when it is enriched in short wavelength energy.^31,37–40

Aside from the timing, dose, and spectrum of photic exposure, the relative position and directionality of light entering the eyes can also have an impact. Melatonin suppression and phase resetting by light have been shown to depend on the region of the retina exposed in humans, with significantly greater photic responses with inferior and nasal retinal illumination.^41–45

Methods

Measurement of Light and Filtering Properties for Estimating Circadian Input

As a result of these influencing factors, precision in characterizing the biological potency of light for non-visual responses is critical for understanding and optimizing the utility of blue-blocking glasses. Biological potency of light can be quantified by weighting the spectral values by an action spectrum appropriate to the effect under consideration. For example, lighting for visual responses has historically been determined via the standard defined spectral weighting functions for rods (low light, scotopic vision) and cones (bright light, photopic vision). When non-visual physiological effects of light were first observed, there were no defined action spectra for these responses in humans, and consequently, photopic measures of light (e.g., photopic illuminance, in lux) were often used. With the development of action spectra for various non-visual responses and a similar absorption spectra for melanopsin, agreement upon a standard for more precisely quantifying the potency of light for these effects has provided a common and convenient basis for evaluation and comparison.^9,46 Melanopic equivalent daylight illuminance (EDI), reported in lux, is a unit of measurement that differentially weights spectra based on the wavelength sensitivity of the photoreceptors that meditate light for circadian function in humans.⁹ Specifically, the melanopic EDI metric is the illuminance of the standard daylight illuminant (D65) that provides an equivalent melanopic irradiance to the test light source, given by:

$$\begin{array}{ccc}\hfill melanopicEDI& =& \frac{{\int }_{380}^{780}{S}_{mel.}\left(\lambda \right)·Ee,\lambda \left(\lambda \right)·d\lambda }{K^{D^{65}}{}_{mel.,v}}\hfill \end{array}$$

where the spectral irradiance of the source W/m²/nm, Ee,λ(λ) is spectrally weighted with the alpha-opic action spectrum for melanopsin (S_mel.(λ)) and divided by the alpha-opic efficacy of luminous radiation for daylight (D65), $K^{D^{65}}{}_{mel.,v}$ (a constant with a value of 1.3262 lm/W). In terms of melanopic EDI, recent consensus-based recommendations suggest that healthy adults on regular schedules should aim to obtain >250, <10, and <1 lux for daytime, evening, and night, respectively.¹³

By contrast to the above, in typical ad-hoc descriptions, filters such as blue-blocking glasses are often measured using ranges of wavelengths (e.g., 380–460 nm), with a “percent reduction” listed, such as “50%”.⁴⁷ There are a few important improvements that should be made to these methods. First, all wavelengths in any given range do not have the same effect, so we should instead weight a filter at each wavelength, typically based on a known action spectrum or absorption spectrum, such as that of melanopsin.^3,4 Second, for optimal practical utility, a description should provide a singular quantity that meaningfully captures that filter's behavior under typical conditions of use.⁴ Finally, as the physiological effects of light generally scale logarithmically with irradiance,¹³ a logarithmic metric is preferable and avoids potential misinterpretations that might result from use of a linear quantity.

To accomplish this, we refer to the units used for neutral density filters from optics. Following this convention, we can list metrics for filters written instead as optical density (OD), as follows:

$$\begin{array}{c}\hfill OD=-lo{g}_{10}\left(transmittance\right)\end{array}$$

For circadian applications, we define these metrics in terms of the degree to which they would reduce the melanopic irradiance of a “standard” daylight illuminant (D65) via melanopic daylight filtering density (mDFD):

$$\begin{array}{ccc}\hfill mDFD=D_{mel,D65}& =& -lo{g}_{10}(transmittanceofD65,\hfill \\ & & melanopicallyweighted)\hfill \end{array}$$

Written this way, we have a proper weighting for an action spectrum (so that D_mel considers 420 nm light less strongly than 480nm), and we also have a logarithmic metric that more closely approximates a known physiological response. These responses map in a familiar way, where a logarithmic change results in a near-linear (logistic) response. Thus, theoretically, when a filter's density is 1.0, the filter transmits ∼10% of the light in the intended action spectrum, and at 2.0, the filter transmits just ∼1%. Provided these decrements in photic input all fall within the steep portion of the dose-response curve for the particular response in question, they would reduce the biological response in an amount that is proportional to the filter density (e.g., an mDFD = 2.5 for a dose-response spanning 2.5 log units).

We also anchor our measures on the ability of each filter to attenuate D65, as this standard illuminant is a ubiquitously encountered source of light, providing an intuitive meaning to the relevant metrics (Table 1). As an example of relative filtering efficacy, we compare different tints of the same brand (UVEX, Honeywell, Morris Plains, NJ, USA) that vary in filtering capacity, with orange, grey, and amber tints yielding mDFD values of 1.41, 1.04 and 0.28, respectively (Fig. 1). By superimposing those mDFD values upon the consensus dose response function,¹³ we illustrate the shift in melanopic EDI and corresponding response with each of these products. In these examples, the amber tint barely influences melanopic EDI, resulting in a non-visual physiological response very close to the full response with an unfiltered D65 light source at this dose (melanopic EDI of 125 lux). In contrast, the gray tint is a relatively stronger filter, with an mDFD just above 1, reducing the response to the same light source to below 30%. Creating an even greater shift to a lower dose, the orange tint reduces melanopic EDI to <5 lux, with a response of ∼15% of the max, which is just around threshold, meaning it would totally suppress non-visual physiological impacts of light in some individuals and reduce it in most.

Table 1.

Alpha-Opic Illuminances and Filtering Properties of Blue-Blocking Glasses With D65 Light Source

CCT, correlated color temperature; CRI, color rendering index; mDFD, melanopic daylight filtering density.

Figure 1.

Illustration of the effects of filter coatings with differing mDFD values. Shaded regions show the shift in melanopic EDI at the corneal plane under a lighting environment with an unfiltered melanopic EDI of 125 lux, superimposed on the consensus dose response curve for non-visual responses from Brown et al. 2022.¹³

Although we consider mDFD provides the most robust available means of deriving a single quantity upon which to evaluate the benefits of different filters, there are two important caveats to interpreting this (and any similar) approach. First, if the intensity of a given light source is already very high or very low (supramaximal or subthreshold), the actual biological effect of the filter may be small or nonexistent under that condition. Second, because the spectral power distributions (SPDs) of electric light sources differ from daylight, actual performance of any filter could, in principle, be better or worse than the log-reduction specified by our metric.

Specifically, it may be worse if light energy is concentrated in a portion of the spectrum where the filter offers little attenuation, or better if it is concentrated in a portion of the spectrum where the filter offers strong attenuation. To capture these possibilities, we provide comparisons of how our proposed metric relates to the context-specific behaviors of a variety of filters under different commonly-encountered lighting conditions. We further provide insight into likely practical benefits that may be obtained under those various conditions by calculating the predicted magnitude of non-visual responses before and after filtering, using the consensus irradiance response relationship for non-visual responses reported in Brown et al.¹³

Collectively, these analyses reveal that the conditions under which our metrics underestimate filter behavior are largely restricted to narrowband light sources that are not commonly encountered in daily life. Hence, as shown below, for typical white light sources, our measures provide an effective upper-limit to the benefits that might be achieved.

Filtering Properties of Wearable Filters Under Common Lighting Scenarios

We characterized an assortment of wearable filters in the context of this new metric (mDFD) to provide a sense for its utility and to better understand the variability in theoretical efficacy for products currently on the market.

Wearable Filters

Twenty-six different models of eyewear designed to filter light were measured (see Supplementary Material S1 for full manufacturer details). A variety of models were chosen to capture examples spanning the wide range of eyewear that has some filtering capacity and is either (1) explicitly marketed on commercial eyewear websites as “blue-blocking” or as effective for improving sleep, (2) used to block short-wavelength light in vision, sleep, or circadian research, or (3) designed for safety or other applications but effective at reducing the biological potency of light for non-visual responses under certain conditions.

Light Sources

Because individuals are likely to be exposed to a variety of light sources when using blue-blocking glasses as a means to improve sleep and circadian health, we chose six lighting conditions (in addition to the D65 standard illuminant, Table 1) under which to fully quantify the efficacy of the filters for reducing the biological potency of light, spanning several orders of magnitude and representing some of the most likely scenarios that individuals might encounter. SPDs for the D65 standard illuminant and these additional example light sources (described in further detail below) demonstrate significant variability in spectral quality as well (Fig. 2). Using these SPDs and a model of the various filters, we describe how each filter reduces the biological potency of light under several different photic conditions.

Figure 2.

Spectral power distributions for various light sources to which people are commonly exposed. Residential applications include (A) incandescent, (B) iPhone X screen, and (C) a desktop computer monitor. Commercial examples include (D) 5000 K light emitting diode and (E) fluorescent (F11) light sources. Finally, our natural light scenario is (F) outdoor light on a foggy morning in Los Angeles.

Residential Lighting Applications

Residential lighting applications include sources of illumination used in spaces that are owned or rented by individuals to live (and sleep in), and in which the lighting is under the control of habitants, at least to a large extent. Residential lighting is typically lower in intensity as compared to commercial and outdoor lighting, and that is reflected in the intensities selected for our examples.

Incandescent

Although light-emitting diodes (LEDs) are becoming increasingly used in homes (and we characterize an LED light source for commercial spaces, below), incandescent lamps remain a common residential lighting application. Therefore we include an incandescent source of light, as one might have in a living room or bedroom, at 100 photopic lux and 43.47 lux melanopic EDI.

Smartphone Screen

Evening light emitted from smartphone devices is perhaps the situation that applies to the largest number of individuals, as an estimated 58% of adults use screens in the hour preceding sleep (NSF 2022), and this is likewise an area of increasing research.^48–50 Therefore we include light from the iPhone X (Apple, Cupertino, CA, USA), dimmed to 25% (25 photopic lux, 26 lux melanopic EDI at the eye), as an example light source that one might be exposed to in the evening before going to sleep.

Desktop Computer Screen

We also examine filtering glasses combined with light from an un-dimmed, large desktop computer screen (148 photopic lux,140 lux melanopic EDI at the eye) to which one might be exposed while using a word processor in an otherwise darkened room.

Commercial Lighting Applications

Commercial applications may include lighting in work, retail, service, and leisure settings, where people spend time in their day but generally do not sleep. The lighting tends to be relatively brighter than residential settings and, typically, control over the lighting is limited to a select few individuals. There are, however, special instances (e.g., nursing homes) where people spend both their days and nights in such spaces, and these are scenarios in which blue-blocking glasses may be particularly useful if used appropriately. Because work spaces are where a majority of people spend a large proportion of their time and that is mostly out of one's control, we have selected a couple of examples that might be encountered in a typical office setting.

LED

Photic stimuli to which one might be exposed when trying to minimize input at the end of a long work day include energy-efficient LED sources, which are now commonly found in newer office settings. Although color temperature may vary, LEDs in office spaces and retail settings will often be in the cool white range. We have therefore selected a 5000 K LED source with a color-rendering index (CRI) of 83 at 200 photopic lux and 149 lux melanopic EDI as an example.

Fluorescent

Many individuals in a workplace scenario might instead be under fluorescent lighting at a similar photopic intensity and so, we also include a narrow tri- band fluorescent lamp (F11) with a color temperature of 4000° K and a CRI of 82 as an example source at 200 photopic lux and 113 lux melanopic EDI.

Natural Daylight

Although most relevant to night shift workers, wearing blue-blocking glasses during the commute home in the early morning, either just before or after sunrise, as well as at home before sleep, is a highly-studied application. We therefore chose outdoor early-morning light as a final example light source, using specifications for 7 AM on a foggy day in Los Angeles, California (1502 photopic lux, 1770 lux melanopic EDI). The intensity of sunlight spans many orders of magnitude depending on time of day, weather, and location, and this chosen example represents a light source that is brighter than any indoor stimulus but does not approach the maximum possible.

Measurement

A Photo Research PR-655 spectroradiometer (PhotoResearch, North Syracuse, NY, USA) and an X-Rite i1Studio spectrophotometer (X-Rite, Grand Rapids, MI, USA) were held in a fixed position in front of a voltage-stabilized, unfiltered 100 watt halogen light source. Measurements were taken in a completely dark room, and for each filter, the light source was first measured alone; the blue-blocking glasses were then placed in position, and a second measurement was obtained. Plano lenses and filters mounted in spectacles were oriented so that the wearer side was facing the sensor, held as close to the sensor as possible. In cases when the filter had to be held manually, the measurements were repeated several times in order to ensure that operator movements did not influence transmission properties. The filtered SPD was divided by the original unfiltered SPD, once per wavelength. As the meters employed had some stray light error (e.g., the PR-655 has 0.25%), the filtered signal was allowed a small adjustment (∼0.2%, toward zero) to correct for this error, based on the average power measured over the whole spectrum. Data were stored as floating point transmittance values and logged to CSV files for use in later processing.

Data

Based on those measures, the resultant melanopic EDI (in lux) and the predicted non-visual response (as determined from corneal melanopic EDI after filtering and the consensus irradiance response function from Brown et al. 2022¹³ (% maximum)) were derived for the 26 wearable filters in combination with the residential, commercial and natural light sources described above (see Tables 2A–C). Resultant photopic lux and CRIs are also included alongside those measures, to help guide filter selection when vision remains an important consideration. CRI values represent the color rendering of an illuminant formed from a filtered D65 source. Although filters are generally applied to light after it is reflected off surfaces, both filters and surface materials are multiplicative in spectral terms, and therefore the operations can be evaluated in reverse order, which is the method we use. A more comprehensive set of data for each light source can be found in Supplementary Tables S2–S7, with the additional measures of alpha-opic illuminances and the ratio of melanopic to photopic lux content (M/P), where lower values reflect filtering properties that selectively reduce melanopic input. Alpha-opic illuminances, photopic lux, correlated color temperature (CCT), and CRI values were extracted using f.luxometer (https://fluxometer.com).⁵¹

Table 2A.

Filtering Properties of Blue-Blocking Glasses With Residential Light Sources

mFD, melanopic filtering density.

Table 2C.

Filtering Properties of Blue-Blocking Glasses With Natural Light Source: Daylight on a Foggy Morning in Los Angeles, CA

Results

Residential Lighting Applications

In our first residential lighting example, with an incandescent source at 100 photopic lux, 35% of filters result in a melanopic EDI of <10 lux (the evening recommendation),¹³ and approximately half of those still allow for reasonable visual illumination for a living room or bedroom environment (≥50 photopic lux) (Table 2A). Two additional filters allow for high-color rendering, although overall photopic illuminance is relatively low (14 and 31 lux), which illustrates the tradeoffs of different filtering approaches. Specifically, by reducing other wavelengths of the visible spectrum, in addition to the most biologically potent short wavelengths, color perception may remain higher but at the expense of overall brightness in our example set of filters. Light emitted from computer and device screens may vary quite a bit; however, most tested filters are unable to bring the biological potency down to evening recommendations for our example desktop monitor (only 30.8% do). The relatively smaller and dimmer smartphone screen can be reduced to <10 lux melanopic EDI via 57.7% of the measured filters, and this is perhaps the scenario under which blue-blocking glasses could be most easily, most frequently, and most successfully used (Table 2A).

Commercial Lighting Applications

In the office lighting examples, the illumination provided does not meet daytime lighting recommendations (i.e., melanopic EDI of >250 lux),¹³ which unfortunately reflects a common scenario¹; however, the lower melanopic EDI does make it relatively easier for filters to reduce biological potency to recommended evening levels. While there are emerging lighting technologies that would allow daytime recommendations to be met that could also be reduced to <10 lux with the higher-performing filters, such lighting conditions still remain incredibly rare in practice. Under the more common lighting conditions tested here, reductions to melanopic EDI below 10 lux were achieved with approximately 31% and 26% of filters under LED and fluorescent lighting, respectively (Table 2B). Specifically, when looking at the glasses in the “dark orange” range (NOIR ARG, LowBlueLights, UVEX SCT-Orange, Melatonin Shades, Chronoptic, Night Swannies), all six served to meet minimum evening recommendations with an LED light source; however, not all do for the fluorescent (although all come close) (Table 2B).

Table 2B.

Filtering Properties of Blue-Blocking Glasses With Commercial Light Sources

This disparity demonstrates the importance of the spectrum of the ambient light source, because even in two lights that appear quite similar (same photopic lux levels) in the same type of setting, there are glasses that may satisfy recommendations for one light type and not another. Under these office lighting conditions, it is also apparent that some filters do not serve to meet recommendations and yet still greatly reduce photopic lux and thus vision (e.g., UVEX-Espresso and Gray), making these unlikely to be useful in such a workplace environment.

Natural Daylight

In our morning outdoor lighting scenario, when it is encouraged that adults on standard schedules receive melanopic EDI of at least 250 lux,¹³ 46.2% of filters result in biological potencies below this threshold (Table 2C). By contrast, most relevant to night shift workers seeking to minimize circadian effects of morning light, only 7.7% of the filters were able to reduce melanopic EDI to below 10 lux, and those that do largely diminish visual stimulation and/or color rendering capabilities as well. As described earlier, photic intensity is a significant factor in determining biological potency, and if a light is bright enough (such as the sun), it becomes quite difficult to find a filter that can reduce input to <10 lux mEDI, unless there is a significant reduction across a broad range of wavelengths. That being said, the same dark orange tints that perform well with electric lighting conditions still reduce biological potency to below daytime recommendations while supporting vision, and some (though not most) preserve color perception as well (Table 2C).

Practical Conclusions

Taken together, there are some general conclusions that can be made from our data. First, the two darkest, red laser goggles are the only filters that meet the evening recommendation for all five measured light sources; however, they also reduce photopic vision and color-rendering to such an extent that they would be dangerous to wear while performing most tasks (e.g., driving), with an extremely low M/P (close to 0). They might still be considered under circumstances where visual function is not important. For example, similar filters have been used in laboratory studies where nocturnal melatonin was being assayed but participants needed to leave the controlled dark lab environment briefly to use the restroom.⁴⁷

In most situations, however, filters that effectively reduce biological potency while maintaining reasonable visual stimulation will be preferable. For indoor lighting conditions (both residential and commercial), the dark orange-tinted glasses (e.g., Noir ARG, LowBlueLights, UVEX SCT–Orange, Melatonin Shades, Chronoptic, and Night Swannies) meet those criteria. The lenses for these dark orange-tinted glasses (Table 1) result in a melanopic EDI of 1–12 lux, 51–115 photopic lux, and mDFDs between 1.0–2.0 for all tested architectural lighting scenarios (Tables 2A, 2B). These data are consistent with a recent quantification of 50 commercially available blue-blocking glasses wherein “orange” tints were deemed most appropriate for sleep and circadian applications.⁵ Within this group of tints, there is some variability in other metrics of importance as well. For example, when visual performance and color perception are required under low to moderate light levels, lenses with slightly lower filtering efficacy but relatively higher M/P and CRI might be preferable. Among the high-performing, dark-orange lenses tested here, the Chronoptic has a slightly lower mDFD but the highest M/P (0.20) and CRI (>80) of all the blue-blocking glasses in this grouping.

Under low light conditions, particularly those of dimmed smartphone screens, a subset of the filters with an mDFD <1.0 (e.g., light orange, darker yellows) may be sufficient to meet evening recommendations of <10 lux. Yet there is little advantage to using these over the dark orange-tinted glasses, other than the fact that some of these (e.g., Noir ABL, Theraspec, Gamma Ray, 3M Yellow Safety Glasses) will not reduce natural light exposures to below recommended daytime levels if used at the incorrect time of day (Table 2C). Although it may seem obvious to say so, clear or near-clear glasses that are often marketed as “blue blockers” (e.g., Felix Gray, RetinaShield, Gamma Ray Classic, TechShield, Crizal Avance, Crizal Prevencia) do not reduce biological potency sufficiently enough (if at all), even when used with the lowest intensity light from smartphones, making it evident that these glasses do not deserve to be in the same category as the other measured filters.

There is a strong relationship between mDFD and the actual (context-specific) melanopic filtering density across the different examples for all the tested filters. Figure 3A shows how melanopic filtering density under the different example lighting conditions (melanopic [source]FD) compares to filtering density with a D65 standard illuminant (i.e. mDFD). Thus mDFD provides a good approximation of filter behavior across commonly encountered real-world scenarios. This metric also correlates well with the relative benefits those filters might provide under most commonly encountered lighting scenarios, although the actual magnitude of the benefit is very dependent on the dose of the background light (Fig. 3B).

Figure 3.

The mDFD provides a reliable basis to judge the relevant benefits of blue-blocking filters across common use cases. (A) Relationship between mDFD and actual melanopic filtering density for a range of 26 filters across six scenes representing common indoor and outdoor use cases (relationship for each scene analyzed by Spearman correlation). (B) Relationship between mDFD and predicted non-visual response for the filters and common lighting conditions in A, as determined from corneal melanopic EDI after filtering and the consensus irradiance response function from Brown et al.¹³

In addition to helping to select an appropriate commercially-available product for a particular application, this newly-described metric (mDFD) may aid in the development of blue-blocking glasses that more precisely minimize circadian disruption while maximizing vision. As our understanding of the circadian system and biological potency of light in humans continues to evolve, current weighting functions for the various photoreceptors could change, which is in part why we report all of the alpha-opic illuminances (Supplementary Tables S2–S7). Similarly, evidence-based recommendations may be further refined, which may alter the range of filters that are deemed best for particular applications. For now, however, blue-blocking glasses with an mDFD between 1.0–2.0 provide the best balance, reducing biologically potent energy while preserving vision under most lighting conditions.

Additional Considerations for Optimal Implementation of Blue-Blocking Glasses

True blue-blocking glasses, which we define as those having an mDFD ≥1.0, provide a useful tool for modulating light on an individual level, which is a unique advantage over active light-emitting interventions. Light sources are not always within our control, nor can they always meet the different needs of all individuals within a given space. This is true for seemingly straightforward applications, such as use at home just before bed (e.g., spouses who share a bedroom but are on different schedules or have different priorities), as well as more complicated scenarios, such as shiftwork environments (with co-workers who vary in terms of circadian phase, schedule, priorities, and authority). When using blue-blocking glasses to address these situations, however, it is important to consider other relevant factors.

No matter the context, timing is paramount for usage of blue-blocking glasses. Although the solar or social clock time may vary, in most cases, use in the hours immediately preceding desired bedtime is required to reap the sleep and circadian health benefits of blue-blocking glasses. Under naturalistic conditions, with no electric light, humans would be exposed to bright, broad-spectrum light during the day and little or no light at night. However, in reality, most individuals receive inadequate levels of daylight during waking hours and are often exposed to inconsistently-timed light from a wide variety of sources before sleep, potentially disrupting or shifting the biological clock to a later time.^{1,13,18,52–54} Ultimately, if blue-blocking glasses are worn outside of the pre-bedtime period, for any purpose, their usage is not likely to be useful and may even be counterproductive.

Although use in the hours preceding desired sleep is important, simply limiting blue-blocking glasses to this time is not an entirely sufficient prescription, because there may be instances when it is more important to optimize alertness in that same time period, such as while driving or working on tasks that require vigilance. In such cases, blocking short-wavelength light may lead to decrements in alertness, which can result in increased risk of accidents and errors.^6,55 That said, for the clear lenses that are sometimes incorrectly marketed as “blue blockers,” there is likely to be minimal impact on circadian input during the day (i.e., there are no lighting scenarios wherein they would reduce melanopic EDI to any significant degree). Although it is disingenuous to include products with clear lenses in the same category as actual blue-blocking glasses, there is little concern in wearing them during the day for other purposes, beyond the cost of purchasing them, when the evidence for efficacy with other applications is weak.^7,56

Assuming the timing and context of use are appropriate, the filtering strength required to sufficiently reduce photic input may vary under different ambient light conditions. This is demonstrated in our measurement data, wherein a particular type of blue-blocking glasses may be sufficient for most residential applications but will not provide the filtering strength needed under office lighting conditions. In addition, highly effective blue-blocking glasses may result in an across-the-board reduction in input for all physiological effects of light; however, carefully titrated reductions in short wavelength light may also provide a way to fine-tune photic input to target one physiological response without influencing another.^31,57 Thus, although there are general rules of thumb that can be applied to usage of blue-blocking glasses (see Fig. 4), a more nuanced assessment of individual needs may sometimes be necessary.

Figure 4.

Decision tree for use of blue-blocking glasses. Assuming you want to wear blue-blocking glasses for some reason (blue box), black diamonds include relevant questions that must be considered. Depending on the answer to those questions, results include times to not wear blue-blocking glasses (red circles); when to exercise caution (yellow circles); and how to best move forward with using them (green circles)

Independent of filtering properties, the design of blue-blocking glasses can also influence efficacy and uptake. Principally, the degree to which the glasses minimize unfiltered light leakage through to the eye, from the sides, top and bottom, will impact their performance. This is most important for light of greater intensities, such as sunlight on the commute home from work. Typically, glasses that are “wrap-around” or have side panels will perform the best in this regard, although users will want to make sure the fit is snug, because facial features such as ear height, brow ridges, and nose shape can affect the amount of light allowed to enter the eye unfiltered. Other properties of the glasses, such as comfort of nose and ear pieces, profile (how close to the eyes the lenses are), and style features (e.g., color and material of the frame) may impact their efficacy indirectly. For example, if they are not worn because of discomfort or aesthetics, they will not be effective. These “implementation” factors are often overlooked in efficacy studies but remain critically important.⁵⁸

Three recent hybrid effectiveness-implementation studies of multi-component lighting interventions included blue-blocking glasses after work and before sleep (outside of driving home), and assessed ease of use and likelihood of future use of the glasses. Across shift workers using UVEX SCT-Orange glasses in two different wraparound models, S0360X for those who wore corrective eyewear and S1933X for those who did not^31,40 (and unpublished data) (n = 59), these blue-blocking glasses were perceived as easy to use by the majority of participants in three different shiftwork settings (Figs. 5A, 5B). Although ∼80% found them easy to use and ∼56% reported likely future use after the study, reasons provided for lower ease of use scores included mild discomfort or difficulty in remembering to put them on. Reasons for not using them in the future included that they would be difficult to incorporate into schedules; that they don't seem to “work”; that help with sleep is not needed; one individual indicated the yellow tint was problematic for vision; and another indicated incompatibility with a gaming headset. Positive open-ended comments regarding future use included “They help me sleep,” and “I think they are useful as a natural way for sleeping if one has trouble.” Similar to these UVEX glasses, many brands of blue-blocking glasses offer multiple choices of models and styles, making uptake inherently more likely because they can appeal to a variety of preferences. Further implementation data for other tints and models of blue-blocking glasses should be collected in future studies.

Figure 5.

Implementation data from three different shift worker intervention studies using blue-blocking glasses (N = 59), including (A) perceptions of the ease of use of the blue-blocking glasses and (B) the perceived likelihood of future use after the study.^34,35

Finally, although there is a critical need to better support the physiological effects of light when developing lighting solutions, visual illumination still remains an important consideration, particularly when optimal performance is necessary. Blue-blocking glasses reduce input to not just the ipRGCs but the classical photoreceptors as well, leading to potential alterations in visual acuity and color perception. It is important to determine whether color perception is affected in a critical way when filters are applied, particularly for use in certain workplace environments or while operating a vehicle. As an example, if short-wavelength light is differentially filtered, it may prohibit the ability to identify blue light indicators or objects that are green may appear instead as yellow. Furthermore, it is possible that certain blue-blocking glasses could work for some dashboard configurations and not others, making the recommendation of use during driving something that may not be determined simply with current commercially-available models. For most applications, there will be necessary requirements in terms of melanopic EDI, photopic illumination, and color rendering. In addition, activities may be modified to accommodate their use when vision is not sufficiently supported. For example, wearing blue-blocking glasses could be safer when commuting via public transport. Alternatively, although lower in mDFD, sunglasses may be used while driving, and higher mDFD glasses with an orange tint may be worn once an individual has arrived home.

Evidence for Efficacy of Blue-Blocking Glasses for Sleep and Circadian Health

Controlled laboratory studies demonstrate that blue-blocking lenses can successfully minimize the physiological effects of light at night and support better sleep.^59–62 Similarly, field studies have demonstrated the efficacy of blue-blocking glasses in reducing bright daylight before desired bedtimes in shift workers and as a means of minimizing electric light exposure before nocturnal bedtimes in those with regular schedules.

A literature search for interventions utilizing blue-blocking glasses was conducted in February of 2022 and updated in June of 2023. Search terms used in Google Scholar included various forms of the term “blue blockers” (e.g., “blue blockers,” “blue-blockers/ing,” etc.), as well as all specific brands of blue-blocking glasses identified in the resultant literature (e.g., UVEX, Chronoptic). Criteria for inclusion were (1) the study had to use a type of blue-blocking glasses that was still available; (2) the blue-blocking filters had to be eyewear and had to be part of the intervention itself (e.g., not used only to reduce light input for melatonin sampling); (3) a control condition was required; (4) outcome measures needed to include sleep or circadian variables; (5) the study could not be a laboratory-only study; and (6) study participants could not represent a clinical population (other than those with sleep and circadian disorders) or a population for which lighting interventions are contraindicated (e.g., bipolar disorder; ADHD; vision or photosensitivity issues). Reviews, abstracts, theses, and papers not in English were also excluded.

Out of the 99 title-screened publications, 17 were included, and 82 were excluded for the following reasons: eight insufficiently described the blue-blocking glasses used, or they were not currently available; 12 reported use of filters that were not eyewear; 17 used the glasses in a way that was not integral to the intervention; one study did not have a control condition; two had no sleep or circadian outcomes; two were laboratory-only studies; 10 were conducted in clinical populations; 20 were reviews, abstracts, theses, or not in English; and seven were excluded for more than one reason. Finally, three papers describing one study used a product that did not meet our definition of blue-blocking glasses (mDFD ≥1.0). The 17 resultant references describe 16 different field studies that tested the efficacy of true blue-blocking glasses for improving sleep and circadian outcomes in the home or work environment; all but five were published in the last 10 years.

Details about these studies of blue-blocking glasses for sleep and circadian applications are shown in Table 3 (papers from the same study are represented in the same row). The sample populations varied somewhat, with ages ranging from 15 to 71 years old; however, most studies reported an average age within the range of 25–45 years. Four of the studies were in healthy adults with regular schedules^63–66; seven were in shift workers^{31,40,67–71}; four were in individuals with sleep complaints or disorders^72–75; and one was in pregnant women.^76,77 The average sample size was 33, with a range from 4–130. Most of the studies took place entirely outside of the laboratory, although two of them also had in-lab components. Nine of the blue-blocking interventions were stand-alone interventions, consisting entirely of using blue-blocking eyewear, whereas seven of the studies were multicomponent interventions, with the glasses as only one part of the intervention (most commonly alongside a bright light intervention during waking hours). Many (10/16) of these studies had participants use the blue-blocking glasses at night before bed, which is timed appropriately for reducing input to the circadian system in those on regular schedules. The duration of time the glasses were used varied considerably, however, from 1.5 to more than four hours before sleep. Thus these blue-blocking interventions sometimes resulted in deviations from recent recommendations for evening photic exposures in healthy adults on standard schedules, which specifies that light should be reduced to <10 lux melanopic EDI beginning at least three hours before sleep.¹³ Of the remaining six studies, which used blue-blocking glasses during daytime hours in those working night shifts, instructions for time of use differed, with some studies instructing participants to wear them throughout the day (both outside and inside),^67,68,71 and others just before sleep.^31,40,69 Only two studies explicitly told individuals not to drive while wearing the blue-blocking glasses,^31,40 and another cautioned participants to remove the glasses if they felt “sleepier than usual, especially while driving.”⁶⁷ Conversely, some study instructions for use explicitly included wearing “while driving a car”⁷¹ and “during the commute,,^69,72 although in the latter cases, it was not specified whether that was while driving, carpooling, or taking public transport. As discussed elsewhere in this article, there are potential safety concerns when wearing blue-blocking glasses while driving, including reduced alertness and impairments in color discrimination. Although no studies to our knowledge have empirically tested the safety of blue-blocking glasses while driving, there were no reported motor vehicle accidents in any of the studies.

Table 3.

Intervention Studies Using Blue-Blocking Glass

Table 3.

Continued

Table 3.

Continued

BIS, Bergen Insomnia Scale; ESS, Epworth Sleepiness Scale; ISI, Insomnia Severity Index; KSD, Karolinska Sleepiness Diary; KSS, Karolinska Sleepiness Scale; PIPR, Post-Illumination Pupil Response; PIRS, Pittsburgh Insomnia Rating Scale; PSAS, Pre-Sleep Arousal Scale; PSD, Pittsburgh Sleep Diary; PSG, polysomnography; PSQI, Pittsburgh Sleep Quality Index; PVT, Psychomotor Vigilance Task; SDI, Sleep Disorders Inventory.

^*Measured in a laboratory.

Brands and tints of blue-blocking glasses varied across these studies, but 15/16 studies utilized the dark orange-tinted glasses (mDFD of ≥1.34, see Table 1), and all used wraparound styles. In terms of specific models, nine of the studies used UVEX brand eyewear to block blue light (eight SCT-Orange tint, one SCT-gray). The next most common brand was Chronoptic (four studies), followed by LowBlueLights, NOIR, and Swanwick brands (two, one, and one study(ies), respectively). One study used both UVEX SCT-Orange and Chronoptic. As described above, blue-blocking glasses in this grouping tend to most effectively reduce melanopic input while still maintaining reasonable visual stimulation under office lighting conditions (melanopic EDI of 2–12 lux and photopic lux 73–113 under LED and fluorescent lighting conditions of 200 photopic lux). Regarding control conditions, eight of the studies used a different tint of eyewear as their control (five clear, three another color), whereas eight studies used no eyewear for their control condition.

There was significant overlap in the outcomes assessed in the various studies, though methods and outputs were not identical. All studies measured sleep in some way (e.g., actigraphy, diary, and questionnaires); eight also assayed circulating levels of the pineal hormone melatonin; and most also examined some combination of one or more of the following additional variables: alertness, performance, light, or mood. Across all studies, at least one outcome was statistically improved for the intervention versus the control condition. Cumulatively, results indicated improved sleep timing and melatonin patterns^{31,63–69,71,73}; increased sleep duration and quality^{31,40,65–70,72,75}; and enhanced performance and mood.^{31,40,70,72,74} Although the above studies were selected in part because they focus on measures of efficacy for sleep and circadian outcomes, further information relating to implementation and compliance under real world conditions was also analyzed. In terms of feasibility and uptake, 12 measured implementation factors to some degree (as defined in Harrison et al.⁵⁸). This was primarily via reporting of adherence and/or side effects, although a few studies also included other single-construct measures of use, such as expectations, mood, or comfort/tolerability. Two studies also examined enablers and barriers to use,^31,40 and two included interviews with the participants regarding their use of the blue-blocking glasses.^40,74

True Blue-Blocking Glasses Are Beneficial for Sleep and Circadian Health When Properly Implemented

Overall, there is robust evidence from both basic and applied research for the efficacy of blue-blocking glasses to increase circadian health. In every one of the 16 reviewed studies, wherein appropriately-named blue-blocking glasses (with ≥1.0 mDFD) were used to improve sleep or circadian health in the field, either as a stand-alone intervention or as a single component of a multi-pronged approach, there was some degree of benefit. Notably, in a novel analysis, one review found a near-linear effect of daily use of blue-blocking glasses on sleep quantity, quality, and next-day job performance, whereby each variable improved with continued use over the week.⁷⁰

Findings in these applied studies are consistent with the data from more highly-controlled, laboratory research wherein blue-blocking lenses are capable of mitigating negative effects of mistimed light, improving sleep and performance, facilitating entrainment, and attenuating melatonin suppression by light.^59,60,62 Likewise, these study findings are consistent with research demonstrating efficacy of blue-blocking glasses for improving sleep in clinical populations, which were excluded from our review.^78–81 Recent syntheses of the literature similarly found evidence in support of the efficacy of blue-blocking glasses for sleep and circadian health,^82,83 though not overwhelmingly so.^7,84 Failure to find strong, consistent effects across, and even within, some of these recent literature reviews may therefore have more to do with ill-defined categories of eyewear due to lack of, suboptimal, or underutilized metrics for characterizing the filtering capabilities of blue-blocking glasses⁷ (e.g., including clear or near-clear coatings), or overly-stringent inclusion criteria for studies (e.g., both RCTs and large sample sizes are difficult to achieve, and therefore rare, in applied lighting intervention studies; two reviews defined improvements in sleep and circadian health via measures of sleep quality alone, rather than duration, timing, etc.).^7,84

Conclusions and Recommendations

We have characterized 26 commercially-available glasses that have been used or referred to as “blue-blocking glasses” under several common lighting scenarios to demonstrate the resultant variability in terms of both melanopic and visual input. Not only do “blue-blocking glasses” vary wildly in their filtering properties (mDFD ranging from 0.01–3.43), but some are even totally undeserving of the term. Our new proposed metric allows for a more objective assessment of the potential utility of wearable filters for specific lighting applications. Combining our theoretical findings with results from intervention studies of blue-blocking glasses, only products that achieve a mDFD ≥1.0 are capable of reducing the biological potency of light to a meaningful extent under most common lighting conditions. This threshold should be considered for both future research as well as practical applications, taking into account the context of target aims. Furthermore, those clear or near-clear glasses that do not reduce the biological potency of light in a meaningful way should not be marketed as advantageous for sleep or circadian health, as some currently are, because there is no evidence to support this.

When using blue-blocking glasses to address sleep and circadian health, elements mediating light other than wavelength must also be considered. For example, although non-visual physiological effects of light are generally most sensitive to short wavelength blue light, other wavelengths of light also provide input to the system. Thus a bright enough light can still elicit a response, even in the absence of the most potent blue energy region of the spectrum. This was observed in our filter measures with morning sunlight wherein very few wearable filters were capable of reducing melanopic input to evening recommendations, and those that did had commensurate decreases in visual stimulation.

Most current architectural lighting applications were designed with only the visual system in mind and thus include a disproportionate amount of longer wavelength light. With this suboptimal spectral quality, reasonably high intensities of light are required to achieve the 250 lux melanopic EDI daytime recommendation. Under such high photic intensities, it is not possible to reduce light levels to <10 lux melanopic EDI in the evening via most of the blue-blocking glasses we measured (other than the laser goggles), as a significant amount of light remains unfiltered. Even when it is not possible to meet both day and evening recommendations, increasing the contrast in biological potency at these two different biological times of day is still likely to be beneficial.^1,18 In addition, other tools such as darkout curtains, screen filters, and sleep masks may further reduce photic input in the time preceding desired bedtimes.

It is also worth noting that integrative lighting solutions are evolving rapidly. Specifically, spectral engineering is now being used to develop novel LED technologies that are supportive of both visual and non-visual physiological properties of light while still maintaining efficiency standards.³¹ Thus in the future it will become increasingly possible to use the same lighting at all times of day, when combined with blue-blocking glasses, to better support health and safety. This strategy will be particularly important in shared spaces that are used for both daytime and evening activities, such as inpatient clinical settings; communal residential facilities (e.g., rehabilitation clinics, nursing homes); and operational living environments (e.g., submarines, space shuttles).

With true blue-blocking glasses, there remain additional considerations surrounding misuse, particularly in terms of timing. Under most conditions, it would only be desirable to reduce short wavelength light close to desired bedtimes. Actual blue-blocking glasses are capable of reducing natural light input to an extent that results in a failure to meet consensus-based daytime recommendations, akin to those who spend most of their daylight hours indoors under electric lighting, which is both relatively less intense and typically contains a lower proportion of short-wavelength energy.^1,13 Even without filters, the predominance of electric light exposure results in many people obtaining insufficient daytime input,⁵³ and the use of blue-blocking glasses during the day will only exacerbate this issue. Furthermore, wearing blue-blocking glasses may be counterproductive while working or when participating in any activity where reduced alertness could jeopardize safety.

Driving while wearing blue-blocking glasses poses a particular concern. Although intervention studies using blue-blocking glasses have not reported increased accidents or errors, the increasingly widespread usage of these wearable filters warrants empirical testing of their safety. There were disparities around the instructions for blue-blocking glasses in the studies we reviewed, most notably surrounding wear while driving, with some investigators explicitly avoiding their use while driving^39,40 and others specifically requiring it.^67,71 Although there are investigators who believe particular models to be safe for driving,^61,85 concerns have been raised.⁶ In addition to potential reductions in alertness and performance, the degree of fatigue and circadian phase of the driver will also contribute to safety and efficacy of blue-blocking glasses. This is especially true for night workers, who are one of the populations for which blue-blocking glasses may be most beneficial. The commute home after a night shift typically coincides with an ideal time for reducing melanopic input, although, as shown in Table 2C, most true blue-blocking glasses still only reduce the potency of daylight by about half (∼40–70%), to a questionably-meaningful degree. In addition, from a safety standpoint, high visual acuity and color perception are crucial for driving; thus the impact of the blue-blocking glasses on visual performance is an important consideration for such applications as well.

Studies to date demonstrate the potential benefits of blue-blocking glasses as an individualized tool for manipulating lighting to better support sleep and circadian health, although further investigation is required. First, although some studies measured or reported filtering properties, most did not, and many tested wearable filters could not even be measured by us because full manufacturing information was not provided or the products were no longer available. Studies of lighting interventions, including those using blue-blocking glasses, should follow recent reporting guidelines for light.⁸⁶ In addition, calculating and reporting mDFD of blue-blocking glasses will provide a clearly defined and intuitive metric for which to assess the filtering efficacy of products used in research studies and marketed to the public.

Finally, individual guidance and education are critically important to ensure the responsible use of these tools. Evidence-based programs that specifically focus on circadian health and lighting can be particularly effective for helping individuals recognize the importance of their light exposure patterns and changing their behaviors in ways that better support sleep and circadian health.⁸⁷ Relatedly, although not the focus of our article, use of blue-blocking glasses for other applications (e.g., macular degeneration, reduction of eye strain) is mainly concerned with reducing blue light during the day, and this may not be compatible with optimizing circadian health, performance, and safety. These different impacts must be weighed, and decisions about blue-blocking glasses may be made, on an individual basis (see Fig. 4). Ultimately, by optimizing the efficacy of blue-blocking glasses for reducing melanopic input, while preserving vision and safety (within the context of individual needs), these short wavelength light-filtering lenses can be an economical and effective tool for better supporting sleep and circadian health.