Ocular light exposure interventions for sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults: An Overview of Cochrane and non‐Cochrane Systematic Reviews

Objectives

This is a protocol for a Cochrane Review (overview). The objectives are as follows:

• To summarise the evidence on the effects of ocular light exposure interventions on sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults.

• To outline opportunities and challenges for developing evidence‐based recommendations for the effective use of ocular light exposure for older adults.

• To highlight areas of remaining uncertainty and gaps in the evidence regarding the effects of ocular light exposure interventions on sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults.

Background

The global population is rapidly ageing, with the number of people over 60 years old projected to almost double from 12% to 22% between 2015 and 2050 [1]. Due to their predisposition to age‐related pathologies, older adults are particularly vulnerable to sleep disturbances, mood disorders, and cognitive decline [2, 3, 4]. These challenges are usually interconnected and can severely impact an ageing individual's daily functioning, quality of life (QoL), and health span [5, 6]. Addressing these challenges is essential for enhancing the well‐being of older adults. Therefore, non‐pharmacological interventions, including ocular light exposure interventions, have become a topic of growing academic and clinical interest as a therapeutic approach.

Over the last two decades, the importance of light for human health and well‐being has become increasingly evident. Light exposure regulates circadian rhythms and sleep‐wake cycles, which are crucial for maintaining overall health. In contrast, 'suboptimal' light exposure has been linked to poor health outcomes, including mood disorders, cognitive decline, fatigue, and sleep disorders [7, 8]. Achieving 'optimal' light exposure levels provides many benefits, including improved sleep quality, circadian rhythm, mood, and cognitive function.

The circadian rhythm is fundamental to nearly every cellular, physiological, and behavioural process in the human body [9]. This makes circadian rhythms not just a peripheral concern, but central to maintaining health and preventing disease. The disruption of these daily rhythms has been implicated in various health conditions, underscoring the importance of maintaining a well‐synchronised circadian clock [9, 10]. This understanding has led to the emergence of circadian medicine, a field grounded in chronobiology that leverages the body’s natural biological rhythms for the prevention, diagnosis, and treatment of diseases [11]. Central to this discipline are interventions like chronotherapy, which involves timing treatments and interventions to align with the body's natural circadian rhythms [11].

Light therapy, a key approach within this field, is a form of chronotherapy. Ocular light exposure therapy relies on the ability of the eyes to receive and process light stimuli aimed at resynchronising the internal clock with the external light‐dark cycle. This approach directly translates basic chronobiological principles into practical medical applications, emphasising the therapeutic potential of ocular light exposure as a non‐pharmacological intervention. Given the pervasive role of circadian rhythms in regulating bodily functions, the vision for the future of circadian medicine is to firmly weave it into standard medical guidelines, ensuring that circadian aspects are always considered and incorporated into treatment plans [12].

Nonetheless, gaps remain in the existing body of literature in our understanding of the effective applications and potential limitations of ocular light exposure interventions. This overview of reviews will summarise the existing evidence on the effects of ocular light exposure interventions on sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults.

Description of the condition

This overview focuses on the non‐visual effects of light on sleep, circadian parameters, rest‐activity cycles, mood, and cognitive function in older adults. Older adults have been observed to experience reduced ocular light exposure attributed to age‐associated changes in the visual system [13], physiological changes in sleep and circadian pathways [14], as well as lifestyle, environmental and health‐related factors [15]. These changes can lead to perturbations in sleep quantity and quality, rest‐activity cycles, mood, and cognitive function, increasing susceptibility to death and morbidity [16].

Age‐associated changes in visual and non‐visual pathways

With age, light admittance into the retina is altered due to structural and functional changes [13]. The lens becomes less flexible and cloudy, leading to presbyopia and cataracts, which affects sharp focus and increases sensitivity to glare. Pupil size and responsiveness decrease, which impacts light adaptation and quality of the retinal image [17]. Simultaneously, the cornea thickens and loses curvature, reducing refractive accuracy. These changes result in decreased visual acuity, contrast sensitivity, and diminished ability to see in low light [13]. Additionally, visual clarity is further impaired as lens yellowing reduces the amount of light entering the eye [18]. Older individuals are also more vulnerable to conditions like dry eye disease, glaucoma, cataracts, and age‐related macular degeneration [13], which can further impair visual and non‐visual functions. These age‐related ocular changes reduce the transmission of light to the retina, potentially disrupting circadian entrainment, sleep, and other non‐visual physiological processes [19].

Age‐associated changes in sleep and circadian physiology

In parallel, and independent of the impact of ocular parameters on the amount of light reaching the retina, there are age‐associated physiological changes in sleep and circadian pathways [14]. These pathways govern the body's internal clock, dictating the rest‐activity cycles and influencing the timing of sleep‐wake cycles and other physiological processes. Changes in these pathways due to ageing contribute to disruptions in sleep patterns, rest‐activity cycles, mood regulation, and cognitive function, affecting the overall well‐being of older adults.

The regulation of sleep and circadian rhythms involves neurobiological mechanisms that undergo significant changes with age. The central circadian clock, located in the suprachiasmatic nuclei of the brain, coordinates these rhythms through the intrinsically photosensitive retinal ganglion cells that transmit light information [20]. The timing and quality of sleep are governed by the interplay of two systems: the sleep‐wake homeostatic drive, reflecting the body's need for sleep based on prior wakefulness, and the internal circadian clock [21]. With advancing age, both systems weaken, decreasing the efficiency of the central master clock and resulting in disrupted synchronisation with the 24‐hour light‐dark cycle, leading to age‐related chronodisruption [22].

This age‐related chronodisruption manifests as alterations in sleep architecture, including reduced slow‐wave sleep and sleep efficiency, increased nighttime awakenings, and a propensity for earlier sleep and wake times [14]. These disruptions contribute to reduced sleep quantity and quality among older adults, resulting in fragmented sleep patterns and daytime sleepiness [22]. While sleep latency shows only minimal increases in healthy ageing, individuals with cognitive impairments often exhibit prolonged sleep‐onset latency, believed to stem from neurodegenerative disruptions in circadian regulation and sleep‐wake homeostasis [23]. Moreover, chronodisruption has broader implications, affecting hormone secretion, thermoregulation, and overall physiological function, thereby impacting the overall health and well‐being of older individuals. For example, reduced melatonin secretion with ageing further exacerbates sleep disturbances and circadian dysregulation [24].

Several brain regions responsible for regulating mood and cognitive function are sensitive to the direct effects of light and to circadian regulation. Light exposure, essential for regulating circadian rhythms and sleep, profoundly affects mood and cognitive function through non‐visual pathways. For instance, retinal light exposure activates the hippocampus and amygdala, depicting the direct influence of light on memory. Imaging studies have also demonstrated that light exposure can affect cortical and subcortical networks associated with cognitive functions, such as arousal, memory, and attention [25, 26].

Furthermore, the circadian and homeostatic sleep‐wake regulating systems interact in distinct ways to affect cognition and mood. Disrupted sleep patterns and shifts in circadian rhythm can significantly impact cognition and mood, especially in older adults. With age, the balance of these systems appears to degrade, impairing cognitive performance [22] and increasing susceptibility to mood disorders such as depression and anxiety [8, 27]. It has been established that the increased prevalence of chronodisruption strongly correlates with an increased incidence of mood disorders [28]. This is because key neural systems involved affect regulation, including the limbic brain region and the hypothalamic‐pituitary‐adrenal, operate under circadian control. Disruptions in the circadian rhythm can lead to dysregulated hypothalamic‐pituitary‐adrenal activity, which plays a crucial role in stress response and emotional regulation through cortisol secretion, contributing to the prevalence of cortisol‐associated mood disorders [28]. Moreover, circadian misalignment has been shown to negatively affect cognitive processes, including working memory, vigilance, and alertness. Highly controlled in‐laboratory forced‐desynchrony studies provide further evidence for this relationship, demonstrating that these cognitive functions undergo changes in circadian rhythm and decline as wakefulness is prolonged [29, 30].

Age‐associated changes in lifestyle, environment, and overall health

Disruptions in sleep, circadian rhythms, rest‐activity cycle, mood, cognition and reduced light exposure among older adults are also influenced by lifestyle, environmental, and health‐related factors. Factors such as reduced mobility, poor indoor lighting, and altered sleep patterns often lead to disruption in natural light exposure patterns. Reduced exposure to natural light due to more time spent indoors, lack of physical activity during daylight hours, poor indoor lighting and nighttime light exposure exacerbate these issues [15, 31, 32, 33]. Artificial light from screens and indoor lighting can disrupt circadian rhythms by confusing the brain's perception of day‐night cycles [34]. The suprachiasmatic nuclei cannot distinguish between natural and artificial light, which leads to suppressed melatonin production and disrupted sleep [35]. Moreover, changes in daily activity patterns due to retirement, physical limitations, reduced social engagement, and daytime napping have an effect on the rest‐activity cycles in older adults [15, 36].

The presence of chronic health conditions such as cardiovascular disease, diabetes, and neurodegenerative disorders is also another factor that impacts sleep quality, mood regulation, and cognition, often worsened by polypharmacy, affecting sleep and circadian rhythm [37, 38, 39]. At the same time, a bidirectional relationship is observed whereby chronic perturbations of circadian rest‐activity rhythms have been associated with various adverse health outcomes, including sleep‐wake disorders, some forms of cancer, neurological, psychiatric, cardiovascular, metabolic, immunologic and gastrointestinal diseases [16]. This is similarly observed for Alzheimer’s disease and Alzheimer‐related dementias, where chronodisruption is labelled as both a causative factor and physiological consequence of this neurodegenerative, neuroinflammatory disease [40]. Furthermore, disruptions of the 24‐hour endogenous cycle have been associated with an acceleration of both the rate and severity of Alzheimer’s disease [41]. Fragmentation in sleep‐wake patterns and behavioural dysregulation are also widely prevalent in people with dementia due to impaired circadian function and degeneration of the suprachiasmatic nuclei with this disease [42].

The current body of evidence suggests a multifactorial association of age, sleep, circadian parameters, rest‐activity cycles, mood, and cognition from physiological, environmental, and behavioural standpoints. Hence, understanding the potential of light exposure on these functions will facilitate further understanding and the development of scalable measures for promoting healthy ageing.

Description of the interventions

Ocular light exposure interventions utilise the controlled use of light to achieve specific health benefits. Generally, it involves being exposed to sunlight or certain wavelengths and levels of light for a set period, and sometimes at a specific time of day. Light needs to reach the person's eye for the intervention to be effective, though direct staring at the light source is unnecessary, allowing people to engage in other activities concurrently [43]. Ocular light exposure interventions are classified by several key parameters, including photopic illuminance (measured in lux), melanopic equivalent daylight illuminance (measured in lux), spectral composition and derived metrics, including correlated colour temperature, timing of light exposure (e.g. morning or evening exposure), and duration of exposure. These parameters can be tailored to suit individual needs and target specific outcomes [44]. At present, there is considerable heterogeneity in methods across studies regarding these parameters, and in reporting them [45], with mixed findings [46, 47].

Ocular light exposure interventions can be delivered through various means, such as natural outdoor lighting, lightboxes, dawn simulators, and specialised lighting systems. Timed exposure involves the strategic use of light, either natural or artificial, at specific times of the day. Lightboxes and lamps are portable devices that deliver specific intensities and spectra of light to the user, allowing flexibility in administration. Bright light therapy involves exposure to intense light, typically ranging from 2,500 to 10,000 lux, for a specified period [48]. Bright light therapy aims to reset the circadian rhythm and improve sleep‐wake cycles and is commonly used to treat conditions such as Seasonal Affective Disorder and circadian rhythm sleep disorders [48, 49]. Blue‐enriched light refers to light sources with a relatively higher proportion of short‐wavelength (blue) light, typically peaking around 460 to 480 nm, and is used to enhance alertness and mood [50]. Dawn simulation therapy mimics a natural sunrise by gradually increasing light intensity to gently wake the individual, promoting better mood and alertness [51].

These various methods of ocular light exposure interventions can be tailored to address specific health conditions and adjust the circadian rhythm. Under controlled conditions, they can modulate circadian rhythm timing [52], providing protective benefits in preventing or mitigating age‐related diseases [35]. For instance, morning light exposure can help phase‐advance circadian rhythms, making it easier to fall asleep and wake up earlier. In contrast, evening exposure can delay rhythms for those who need to stay awake longer. Various ocular light exposure interventions, including timed exposure to natural sunlight [53], environmental modifications [54], and bright light therapy [47], have shown promise in improving sleep quality, cognitive function, and mood in older adults.

Adverse effects of interventions

Potential adverse effects of ocular light exposure interventions include eye strain, headaches, nausea, fatigue, and agitation, which are usually mild and temporary, often part of an adjustment phase. These responses tend to occur more frequently in studies where the light source is positioned close to the eyes, such as with head‐mounted visors [55]. Few studies have recorded adverse reactions from ocular light exposure interventions, even among older adults [56]. To illustrate, Chang and colleagues reported in their review that none of the 395 older adult participants experienced adverse effects from bright light therapy [57]. However, older individuals taking photosensitising drugs or with existing retinal damage may be contraindicated and should seek an ophthalmology evaluation before undergoing ocular light exposure interventions due to higher potential risks [58]. Considering these exceptions, ocular light exposure interventions appear to offer a good balance between risks and benefits. Despite these minor risks, they show great promise as an alternative to psychotropic drugs for treating sleep and mood disorders [59]. To maximise the benefits and minimise any adverse effects, ocular light exposure interventions should be tailored to individual needs, with careful consideration given to light exposure parameters.

How the interventions might work

The mechanism behind ocular light exposure interventions lies in the non‐image‐forming effects of light, mediated by specialised, intrinsically photosensitive retinal ganglion cell photoreceptors in the eye [60]. These cells, which contain the light‐sensitive pigment melanopsin, play a crucial role in regulating circadian, neuroendocrine, and neurobehavioural functions [61]. Light has acute effects on non‐image‐forming functions, such as circadian rhythms, sleep, mood, and cognitive processes [61]. When light enters the eye, it is detected by intrinsically photosensitive retinal ganglion cells, which then send signals to the suprachiasmatic nuclei in the hypothalamus, the brain's master clock that coordinates daily physiological rhythms [60]. The non‐image‐forming system influences various physiological responses to light through direct projections from intrinsically photosensitive retinal ganglion cells to subcortical areas such as the suprachiasmatic nuclei, the intergeniculate leaflet, and the olivary pretectal nucleus [60]. These physiological reactions to light depend on several variables, including the timing and duration of exposure, prior light exposure, and the intensity and spectral composition of the light [45]. Light serves as the primary conduit that entrains the circadian system to the 24‐hour day‐night cycle [34]. By optimising light intensity, spectrum, timing, and duration, ocular light exposure interventions aim to reset the circadian clock and influence neurochemical processes in the brain [52, 58].

As described above, older adults often experience reduced sensitivity to general sensory input, neurobiological alterations in circadian and sleep systems, and decreased exposure to bright environmental light. Additionally, they exhibit reduced sensitivity to the effects of light on the suprachiasmatic nuclei. This issue is particularly pronounced in those with chronic diseases or neurodegenerative conditions [38, 40]. Ocular light exposure interventions can address these challenges by providing additional or alternative light sources, promoting the synchronisation of internal circadian rhythms with environmental light‐dark cycles. This synchronisation can help mitigate sleep disturbances, mood disorders, and cognitive decline commonly seen in older adults, thereby enhancing their overall QoL. Additionally, they can provide benefits through the direct effects of light, such as improved alertness, mood enhancement, and better cognitive performance.

Why it is important to do this overview

The ageing population is growing faster than ever, bringing with it the need to manage the health challenges faced by older adults. Older adults are increasingly vulnerable to disruptions in sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function, which heightens their risk of adverse health outcomes. Tackling these issues is crucial for promoting healthy ageing and enhancing the QoL in older adults. There is a critical need for effective, non‐pharmacological interventions, supported by the rising recognition of environmental factors in shaping health outcomes in older adults [62]. Ocular light exposure interventions can be considered due to their non‐invasive nature, feasibility, affordability, and minimal risk [55]. They can optimise the QoL for older adults by positively impacting sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function.

Despite their potential, research on ocular light exposure interventions remains fragmented across various systematic reviews, resulting in a lack of consensus on effective practices. Variations in methodologies in existing studies have hindered the formulation of clear, evidence‐based recommendations. Current practice shows positive results in ocular light exposure interventions based on a set of factors: bright 'white' light, intensities ranging from 2,500 to 10,000 lux for 30 minutes to two hours daily, sustained over two to four weeks [46, 63]. Given the heterogeneity in delivery and outcomes, it is necessary to comprehensively examine all forms and methods of ocular light exposure interventions, focusing on their effects on sleep quantity and quality, rest‐activity cycles, mood, and cognitive function. Determining the optimal light levels and other parameters for effective ocular light exposure interventions is necessary to provide actionable guidelines.

Over the past few decades, there has been a growing interest in the therapeutic potential of light exposure. While much has been documented about its therapeutic uses, optimal characteristics of light that affect health and well‐being, including dose and timing, are not yet fully understood. Foremost, human‐centric lighting is emerging, emphasising and demonstrating the positive benefits of integrative lighting for physiological and psychological well‐being [64, 65]. In 2022, Brown and colleagues published a consensus paper proposing targeting light exposure levels for day‐active, healthy adults [66]. However, these recommendations did not capture the non‐visual effects of light during ageing. Considering the rapidly growing population of individuals aged 65 years and above, closing this knowledge gap is crucial. Subsequently, recent priority‐setting initiatives have focused on developing policy‐relevant evidence based on the non‐visual effects of light on human health. In 2023, the UK Parliament's House of Lords Science and Technology Committee highlighted the need for more research on light pollution at night and its impact on sleep and circadian rhythms [67]. The committee's report recommended strengthening the evidence base to facilitate the translation of findings into actionable policies and interventions [68].

This overview of reviews is timely, aiming to consolidate and advance the evidence base to catalyse actionable interventions. This overview aligns with the Cochrane Handbook for Systematic Review of Interventions classification of overviews that synthesise evidence from reviews of different interventions for the same condition or population [69]. In this case, older adults are exposed to various forms of ocular light intervention. We aim to synthesise findings from the existing body of evidence of ocular light exposure interventions applied to older adults, specifically evaluating the effectiveness of different approaches, including artificial light sources and natural daylight exposure, on sleep quantity and quality, circadian rhythms, rest‐activity cycles, mood, and cognitive function. The overview also aims to outline evidence‐based recommendations for healthcare providers and researchers, clarify methodological approaches, and highlight areas requiring further investigation. Addressing the gaps in current knowledge will further guide future studies and enhance our understanding of how best to optimise ocular light exposure interventions for healthy ageing. The findings of this overview may inform policy development and guide ageing‐in‐place initiatives and advancements in lighting technology tailored to the needs of older adults. Ultimately, by harnessing the potential of ocular light exposure interventions for sleep, circadian regulation, rest‐activity patterns, mood and cognition, this overview seeks to promote QoL and resilience among older populations.

While we acknowledge the inherent heterogeneity within older adult populations, which includes differences in cognitive status, health conditions, and living environments, this overview intentionally reflects the diversity of real‐world ageing populations. Given the broad application of ocular light exposure interventions across various clinical and community settings, synthesising evidence across these subgroups remains necessary. Where data allow, we will stratify findings by cognitive health status (e.g. no cognitive impairment, mild cognitive impairment, dementia), mode of intervention delivery (e.g. institutional versus community), and other relevant characteristics to explore potential sources of heterogeneity. This approach is consistent with best practices in overviews that aim to capture translational value across varied populations and settings [69]. This overview aims to ensure that meaningful conclusions can still be drawn while acknowledging variability in the population and interventions.

Objectives

• To summarise the evidence on the effects of ocular light exposure interventions on sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults.

• To outline opportunities and challenges for developing evidence‐based recommendations for the effective use of ocular light exposure for older adults.

• To highlight areas of remaining uncertainty and gaps in the evidence regarding the effects of ocular light exposure interventions on sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults.

Methods

This overview will be conducted in full alignment with Chapter V of the Cochrane Handbook for Systematic Review of Interventions [69].

Criteria for considering reviews for inclusion

Types of reviews

We will include Cochrane and non‐Cochrane systematic reviews that meet the inclusion criteria, with or without meta‐analyses. We will include systematic reviews of variable study designs, namely randomised controlled trials (RCTs) and non‐RCTs (e.g. quasi‐experimental, observational studies), provided they report results separately by study design. We will extract and present findings from RCTs independently of non‐RCTs. Reviews that combine results from different study designs without distinguishing them will be excluded from our synthesis. Including reviews from both Cochrane and non‐Cochrane sources presents the challenge of overlapping reviews. Hence, when two non‐Cochrane reviews evaluate the same treatments and examine the same outcomes, the most recent and methodologically robust review will be included. If reviews differ in the outcomes assessed, both reviews may be included. We will note and exclude any Cochrane systematic reviews that have been withdrawn from the Cochrane Library.

Types of participants

We will include systematic reviews of trials including older adults, defined as individuals aged 60 years or older [70], of any sex, any state of health, living within the community or in long‐term care settings. We will exclude systematic reviews that include studies of participants younger than 60 years.

Given that ocular light exposure is a niche and emerging field, particularly in the context of older adult populations, imposing overly narrow eligibility criteria (e.g. by restricting to only healthy older adults or specific residential settings) would likely result in very few, if any, eligible systematic reviews. To ensure feasibility while still maintaining rigour, we have opted for broader inclusion criteria that reflect how these interventions are applied across various settings.

Types of intervention

We will include systematic reviews that include trials of any form of ocular light exposure intervention used to address sleep, circadian rhythms, rest‐activity cycles, mood, and cognitive function in older adults. This includes, but is not limited to, outdoor daylight exposure, indoor daylight exposure, simulated environmental light exposure such as dawn‐dusk simulation therapy, bright light therapy, phototherapy, light boxes, and light‐emitting diode lighting systems for fixed or dynamic ambient room lighting. All variants of doses, delivery methods, duration, illuminance levels, and modes of intervention will be considered.

Types of comparison

We will include systematic reviews that include trials evaluating comparisons between ocular light exposure interventions and 1) placebo or sham interventions, 2) no intervention or continuation of usual care, or 3) other forms of interventions alongside ocular light exposure.

Types of outcome measures

We will include systematic reviews that evaluate at least one outcome measure sensitive to changes in sleep quality and quantity, circadian parameters, rest‐activity cycles, mood, and cognitive function in older adults, with no restrictions on time of measurement. Reviews that intended to report on at least one of these outcomes but found no data will also be included.

Primary outcomes

• Sleep quantity or quality or both: measured by actigraphy measurement or self‐report scales such as the Pittsburgh Sleep Quality Index [71], sleep charts, Epworth Sleepiness Scale [72], or other validated measures.

• Sleep architecture: measured with polysomnography or other validated measures.

• Circadian parameters: measured by serum melatonin levels, saliva cortisol profile, skin temperature, or other validated measures.

• Behaviour rest‐activity cycles: measured by actigraphy or self‐report scales such as the Short Scale for Sleep‐Wake Disturbances [73], or other validated measures.

• Mood disorders: measured by self‐report scales such as the Geriatric Depression Scale [74], Positive and Negative Affect Schedule [75], Philadelphia Geriatric Centre Morale Scale [76], Patient Health Questionnaire‐9 [77], or other validated measures.

• Cognitive function: measured through instruments such as the Mini‐Mental State Examination [78], Clinical Dementia Rating Scale [79], or other validated measures of single or multiple domains of cognition.

Secondary outcomes

• Disorders associated or co‐existing with mood disorders that include but are not limited to behavioural, psychological, and psychiatric disturbances (e.g. anxiety, agitation, delusions, hallucinations).

• Any other reported outcomes, such as QoL (e.g. QoL, psychosocial wellbeing) and/or functional ability (e.g. activities of daily living, risk of falls).

• Safety of intervention and adverse events associated with ocular light exposure interventions, as reported in the reviews.

Hierarchy of outcome measures

Where multiple outcome measures are reported for the same outcome domain within a review, we will apply a hierarchy of preference to guide data extraction as outlined below.

• Validated and widely used instruments, recognised in clinical or research contexts for their reliability and comparability (e.g. Pittsburgh Sleep Quality Index [71], Mini‐Mental State Examination [78], Geriatric Depression Scale [74]).

• Objective measures, such as actigraphy, polysomnography, or biological markers (e.g., melatonin, cortisol), particularly when these offer greater precision or reduce the risk of bias compared to self‐report.

• Primary outcomes as specified by the original systematic review.

• The most frequently reported instrument across included reviews to enhance consistency and comparability in synthesis.

If multiple equally appropriate measures are reported without clear prioritisation, all relevant data will be extracted and presented narratively.

Search methods for identification of reviews

We will identify published systematic reviews in any language, with no restriction by date. The development and execution of the search strategy is conducted with the support and expert guidance of a Cochrane Information Specialist (CB), who ensures that the strategy is comprehensive, methodologically sound, and aligned with Cochrane standards.

As a preliminary step towards establishing the search strategy for this review, three domain experts (RRM, ZAA, MS) engaged in an intensive brainstorming session to collaboratively develop a comprehensive table (see Supplementary material 1). This table encompasses an extensive list of potential keywords and terms, supported by a detailed thesaurus, including truncation and wildcards to ensure the inclusion of relevant terminology beyond standard Medical Subject Headings (MeSH) terms. This approach was meticulously designed to capture all possible systematic reviews, addressing variations in terminology and nomenclature across different studies informed by the existing body of evidence on ocular light exposure.

Electronic searches

We will search the following databases for relevant systematic reviews.

• MEDLINE via Ovid (from 1946 onwards)

• Embase via Ovid (from 1974 onwards)

• The Cochrane Database of Systematic Reviews (CDSR) via the Cochrane Library (most recent issue)

• Cochrane Central Register of Controlled Trials (CENTRAL; most recent issue) via the Cochrane Library.

Additionally, we will also search the following trial registries to identify ongoing or planned trials that may not yet be included in systematic reviews.

• ClinicalTrials.gov (from 2000 onwards)

• World Health Organization (WHO) International Clinical Trial Registry Platform (ICTRP) (from 2005 onwards).

Records identified through these trial registries will not be included in the formal synthesis of the selected reviews. However, they will be described narratively to highlight potential gaps in the current evidence base and to inform future research priorities.​

Our search strategies will employ a combination of MeSH terms and keywords to search these databases (see Supplementary material 1). Additionally, we applied a seed review validation process to ensure the robustness of our search strategy and its ability to capture relevant literature. Three established systematic reviews were used as benchmarks to verify the comprehensiveness and sensitivity of the search strategy [46, 80, 81]. The search strategy includes a combination of MeSH terms and free‐text terms (see Supplementary material 1).

We will also apply the systematic review filter (see Supplementary material 1) developed to identify systematic reviews in MEDLINE (Ovid) outlined by the University of Pittsburgh Health Sciences Library System Guide [82, 83]. We will manage the retrieved citations using Zotero. Where the scope of an included systematic review extends beyond the parameters of this overview, such as including broader populations, settings, or outcomes, we will extract and synthesise data exclusively from the subset of primary studies that align with our predefined eligibility criteria. If the relevant subset of primary studies cannot be clearly identified, the review will be excluded from the synthesis to ensure conceptual consistency and methodological clarity.

Searching other resources

Searching reference lists

We will check the reference lists of all included systematic reviews to identify further references to relevant reviews.

Correspondence with experts

We will contact the original authors for clarification and further data if needed.

Errata, retractions and expressions of concern

We will run a specific search to capture if any included reviews or their primary studies have been subject to post‐publication amendments. These include published errata, retractions, and expressions of concern. To identify such amendments, we will:

• Search the Retraction Watch Database

• Check Ovid for publication types such as “Retracted Publications”, “Published Erratum” and “Expression of Concern”

• Review journal websites for any post‐publication notices linked to included articles.

Any reviews or primary studies identified with post‐publication amendments will be clearly documented in the review. We will consider their potential impact on the review’s conclusions as part of our narrative synthesis and risk of bias assessment, rather than applying automatic exclusion.

Data collection and analysis

Selection of reviews

Two review authors (RRM, ZAA) will independently screen all retrieved reviews at the title, abstract, and full‐text stage with respect to the inclusion criteria (see Criteria for considering reviews for inclusion). Discrepancies will be resolved through discussion or, if needed, consultation with a third review author (MS).

We will ensure that only the most recent version is included for reviews sourced from the CDSR. In instances where the CDSR retrieves a protocol or an insufficiently up‐to‐date review, we will contact the Cochrane Community Support and the authors of the reviews to request pre‐publication versions or updated reviews of interventions, which can be assessed for inclusion in the review. All screening decisions will be documented, and reasons for exclusion at the full‐text stage will be recorded and reported in a PRISMA flow diagram [84]. All potentially eligible systematic reviews will be included during the title and abstract screening stage, without considering methodological quality.

Data extraction and management

A table mapping the primary studies will be included to identify any overlap between the reviews (see Supplementary material 2). We will extract and present data in a standardised data extraction form. We will develop a table to outline the characteristics of the included reviews, including the title, author, publication date, number of studies, number of participants, population, intervention, control, outcomes, and review limitations (see Supplementary material 3). A summary of findings table will also be developed using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) assessment (see Assessment of methodological quality of included reviews). The two review authors will independently extract and cross‐check data, with any differences resolved by a third review author (MS). We will contact systematic review authors if clarification is required, if information is ambiguous, or unavailable in the review. We will not re‐extract information from the original studies included in the systematic reviews.

Assessment of methodological quality of included reviews

Quality of the included systematic reviews

We will assess the quality of the included systematic reviews using the AMSTAR‐2 (A Measurement Tool to Assess Systematic Reviews), a critical appraisal tool for systematic reviews that include RCTs or non‐RCTs of healthcare interventions [85]. AMSTAR‐2 comprises 16 items designed to provide a comprehensive assessment of the quality of systematic reviews and identify critical weaknesses that compromise confidence in the reported findings. We will develop a judgement of confidence based on the AMSTAR‐2 with specific attention to the following critical domains.

• Protocol registration (item 2)

• Adequacy of the literature search (item 4)

• Justification for excluding individual studies (item 7)

• Risk of bias assessment of included studies (item 9)

• Appropriateness of meta‐analytic methods (item 11)

• Consideration of risk of bias when interpreting results (item 13)

• Assessment of publication bias (item 15).

Each systematic review will receive an overall rating of ‘high’, ‘moderate’, ‘low’, or ‘critically low’ confidence in the results, depending on the number of critical flaws and/or non‐critical weaknesses. Results of the AMSTAR‐2 appraisal will be summarised in a table (see Supplementary material 4), including item‐by‐item responses and justifications for overall confidence ratings.

Risk of bias of the primary studies included in the systematic reviews

We will assess the risk of bias in the included reviews using the risk of bias in systematic reviews instrument (ROBIS) [86]. All reviews that meet the inclusion criteria will be retained in the overview, regardless of the ROBIS assessment judgement. Reviews assessed as having a high risk of bias will be clearly identified and take into account the methodological limitations. The ROBIS assessment process will be conducted in three phases:

Phase 1: relevance to the overview question. This initial phase evaluates whether the review is appropriate in scope and aligns with the research question of the overview.

Phase 2: identification of concerns in the review process. This phase consists of four domains.

• Study eligibility criteria

• Identification and selection of studies

• Data collection and study appraisal

• Synthesis and findings.

Phase 3: overall risk of bias judgement. Each domain includes five to six signalling questions answered with one of the following options: ‘yes’, ‘probably yes’, ‘probably no’, ‘no’, or ‘no information’. Based on these responses, the overall level of concern for each domain will be judged as ‘low’, ‘high’, or ‘unclear’. If most responses are ‘yes’ or ‘probably yes’, the domain will be rated as low concern; if one or more responses are ‘no’ or ‘probably no’, the domain will be rated as high concern. In this final phase, the overall risk of bias for the review is judged based on summarised concerns from Phase 2 and three final signalling questions. Similar to Phase 2, if responses are mostly ‘yes’ or ‘probably yes’, the overall risk of bias will be considered low. If one or more responses are no’ or ‘probably no’, the overall risk will be rated as high.

Two review authors (RRM, ZAA) will independently apply the ROBIS tool to each included review. Any discrepancies will be resolved through discussion or, if necessary, by consultation with a third review author (MS). The results of the ROBIS assessment will be summarised and presented in a table (see Supplementary material 5).

Quality of the evidence in the included reviews

The GRADE tool will be used to assess the certainty of evidence for the included reviews [87]. The GRADE approach determines the level of certainty considering risk of bias, imprecision, inconsistency, indirectness, and publication bias. GRADE has four evidence certainty or quality levels – very low, low, moderate, and high – which will be independently assessed and determined by two review authors (RRM, ZAA). Discrepancies will be resolved through consensus with a third review author (MS). Where the included systematic reviews have utilised GRADE, the review authors' GRADE evaluations will be presented and factored into interpreting the results. In cases where the included reviews have not used GRADE, we will rely on the quality assessments of the primary studies as provided in each review.

The following outcomes are prioritised for GRADE assessment based on their clinical relevance to the objective of this review.

• Sleep

• Circadian rhythm

• Behaviour rest‐activity cycles

• Mood

• Cognitive function.

Currently, there is no consensus on the definition of timeframes for the impact of light on human physiology, as its effects are known to occur across multiple time scales. In this overview, we adopted an operational threshold of one week to distinguish between acute and chronic effects. Interventions lasting less than one week were classified as targeting acute (short‐term) effects, which may manifest over minutes to days. Conversely, interventions lasting more than one week were considered to target chronic (medium‐ to long‐term) effects, typically developing over days, weeks, or months. We will prioritise interventions targeting medium‐ to long‐term effects, as they are more relevant to sustained health outcomes. The GRADE assessment will only be applied to these outcomes and timeframes, and will be reported in the summary of findings table as per the Cochrane Handbook of Systematic Reviews of Interventions [88] (see Supplementary material 6).

Data synthesis

We will provide a comprehensive narrative synthesis of the findings from the included reviews, considering all available data. The detailed characteristics of the included reviews will be detailed in a table (see Supplementary material 3). This table will outline key statistics such as the number of primary studies in each review, participant demographics such as age distribution and sex ratios, as well as inclusion and exclusion criteria applied in each review.

We will explore the various ocular light exposure interventions documented across the reviews. A thorough comparison of the different intervention types and methods will be conducted, including artificial light sources (e.g. fixed lighting systems, lightboxes, dawn simulators) and natural daylight exposure (e.g. outdoor exposure, behavioural modifications). For each intervention category, we will analyse the specific metrics employed, including the dosage of ocular light exposure via illuminance (measured in lux), mode of delivery, duration, frequency of exposure, timing of exposure, and any other relevant parameters. Additional light characteristics such as spectral composition, correlated colour temperature and melanopic equivalent daylight illuminance, will also be extracted where available. We will pay particular attention to identifying both standardisation practices and any notable variations in how these metrics are applied across different studies, as these factors may influence the comparability of outcomes.

Data will be stratified by the following.

• Type of ocular light exposure intervention (e.g. electric light or natural daylight exposure)

• Method of ocular light exposure delivery (e.g. lightboxes, fixed lighting systems, wearable devices, outdoor daylight exposure)

• Setting of delivery (e.g. laboratory, community‐based, institutional)

• Cognitive health status of the population (e.g. no cognitive impairment, mild cognitive impairment, dementia)

• Frequency of ocular light exposure (e.g. session/day or week, user‐selected frequency)

• Duration of ocular light exposure intervention classified as acute (less than one week) or chronic (more than one week)

• Irradiance/illuminance parameters (e.g. photopic illuminance (lux) levels, and melanopic equivalent daylight illuminance (lux) levels)

• Radiance/irradiance parameters (e.g. photopic luminance (cd/m²) levels, and melanopic equivalent daylight illuminance (mW/m²) levels)

• Colour appearance quantities, including correlated colour temperature

• Time of light exposure (e.g. morning, afternoon, evening, or nighttime).

Where feasible, we will map the description of interventions to the ENLIGHT checklist, a consensus instrument for describing stimulus properties of light interventions [45].

We will systematically present data from the systematic reviews without re‐analysing or pooling the data. Findings from systematic reviews that include both RCTs and non‐RCTs will be presented separately as reported in the review. We will also examine the types of comparators used within the reviews, detailing how different interventions were compared against each other or against control conditions (e.g. placebo, no intervention). This analysis will provide insight into the methodological diversity and rigour of the included studies, as well as the robustness of the comparisons made.

Furthermore, we will evaluate the strengths and limitations of each intervention method in terms of its practical application and its impact on various health outcomes. This evaluation will help to contextualise the findings and identify areas where further research may be necessary. We will also clearly delineate and summarise the key points of the overview, emphasising any necessary caution when interpreting the results.

Finally, the summary of findings (see Supplementary material 4) will be reported in accordance with the guidelines outlined in Chapter V of the Cochrane Handbook for Systematic Reviews of Interventions [69]. We will present a summary of findings for the prioritised outcomes as outlined below:

• Ocular light exposure interventions versus placebo or no intervention, or other intervention on sleep

• Ocular light exposure interventions versus placebo or no intervention, or other intervention on circadian parameters

• Ocular light exposure interventions versus placebo or no intervention, or other intervention on rest‐activity cycles

• Ocular light exposure interventions versus placebo or no intervention, or other intervention on mood

• Ocular light exposure interventions versus placebo or no intervention, or other intervention on cognitive function.

In addition, we will also report any other potentially clinically significant outcomes:

• Ocular light exposure interventions versus placebo or no intervention, or other interventions on any other potentially clinically significant reported outcomes.

Supporting Information

Supplementary materials are available with the online version of this article: 10.1002/14651858.CD016157.

Supplementary materials are published alongside the article and contain additional data and information that support or enhance the article. Supplementary materials may not be subject to the same editorial scrutiny as the content of the article and Cochrane has not copyedited, typeset or proofread these materials. The material in these sections has been supplied by the author(s) for publication under a Licence for Publication and the author(s) are solely responsible for the material. Cochrane accordingly gives no representations or warranties of any kind in relation to, and accepts no liability for any reliance on or use of, such material.

Supplementary material 1 Search strategies

Supplementary material 2 Template for table to map the studies within the included systematic reviews

Supplementary material 3 Characteristics of included reviews

Supplementary material 4 AMSTAR‐2 assessment of included reviews

Supplementary material 5 Summary of ROBIS assessment

Supplementary material 6 Summary of findings from included reviews (Higgins 2024)

Supplementary material 7 List of abbreviations

New

Additional information

Contributions of authors

• RRM, ZAA, and MS drafted the protocol. All authors had input into the protocol development and agreed on the final version.

• MS was the contact person with the editorial base and the guarantor.

Declarations of interest

RRM: declares that they have no conflict of interest.

ZAA: declares that they have no conflict of interest.

MS declares the following potential conflicts of interest in the past five years (2021 to 2025).

• Academic roles: Member of the Board of Directors, Society of Light, Rhythms, and Circadian Health (SLRCH); Chair of Joint Technical Committee 20 (JTC20) of the International Commission on Illumination (CIE); Member of the Daylight Academy.

• Remunerated roles: Speaker of the Steering Committee of the Daylight Academy; Ad‐hoc reviewer for the Health and Digital Executive Agency of the European Commission; Ad‐hoc reviewer for the Swedish Research Council; Associate Editor for LEUKOS, journal of the Illuminating Engineering Society.

• Funding: Received research funding and support from the Max Planck Society, Max Planck Foundation, Max Planck Innovation, Technical University of Munich, Wellcome Trust, National Research Foundation Singapore, European Partnership on Metrology, VELUX Foundation, Bayerisch‐Tschechische Hochschulagentur (BTHA), BayFrance (Bayerisch‐Französisches Hochschulzentrum), BayFOR (Bayerische Forschungsallianz), and Reality Labs Research.

• Honoraria for talks: Received honoraria from the Research Foundation of the City University of New York and the Stadt Ebersberg, Museum Wald und Umwelt.

• Patents: Named on European Patent Application EP23159999.4A (“System and method for corneal‐plane physiologically‐relevant light logging with an application to personalized light interventions related to health and well‐being”).

With the exception of the funding source supporting this work, MS declares no influence of the disclosed roles or relationships on the work presented herein.

Sources of support

Internal sources

• Max Planck Society, Germany

  Max Planck Research Group Translational Sensory & Circadian Neuroscience

External sources

• National Research Foundation, Singapore

  National Research Foundation, Singapore (NRF2022‐THE004‐0002)

Registration and protocol

Review topic approved by Cochrane on 8 April 2024.

Data, code and other materials

Data sharing is not applicable to this article as it is a protocol; therefore, no datasets were generated or analysed.

A list of abbreviations used throughout this review can be found in Supplementary material 7.