Can night shift workers benefit from light exposure?

Day‐and‐night cycles have evolved since life on Earth originated 3.8 billion years ago. Plants, humans and animals have an internal biological clock that helps to anticipate and adapt to the regular rhythm of the day. Using fruit flies as a model organism, the discoveries of the 2017 winners of the Nobel Prize in Physiology or Medicine explain how our inner clock adapts our physiology to the dramatically different phases of the day (hours with light and hours with darkness) with exquisite precision.

The health effects of light has gone from being a relatively niche corner of physiology to one of the most talked‐about areas in science in less than two decades. In addition to rod and cone receptors used for vision, the eye contains specialized cells that send non‐image information to the brain: the intrinsically photosensitive retinal ganglion cells (ipRGCs). Not only do the photosensitive cells signal directly to our biological clock regulating critical functions like behaviour, hormone levels, sleep, body temperature and metabolism, but they also signal to several brain regions involved in the regulation of alertness, cognitive performance and mood (LeGates et al. 2014).

We influence our brain with artificial lighting every day. Light in the morning sends a strong awakening signal to our brain. Our inner clock isn't exactly 24 h, it's actually longer, so morning light resets our biological clock every day. Sleep is an interaction between our inner clock and the build‐up need for sleep with time spent awake (sleep homeostasis). However, sleep can be overridden by our behaviour, for instance working night shifts or being exposed to a higher light intensity or light consisting of predominantly short‐wavelengths in the evening/night hours. Reduced light in the evening enables melatonin to be released and gives the proper signal to go to sleep.

Circadian adaptation (a phase shift of the biological clock to align with a shifted schedule like work, sleep etc.), sustained alertness and performance are key elements for those that work at night. However, adapting to night shifts and performing at night is a physiological challenge. Cross‐sectional and longitudinal surveys, field studies and laboratory studies have consistently shown that the circadian adaptation to night shift work improves psychomotor performance and subjective alertness on shift, the daytime sleep following the shift and mood (Kecklund & Axelsson, 2016).

According to light phase response curves, light administered in the evening or during the early part of the night may improve adaptation to night work by delaying the circadian rhythm. In contrast, light administered in the morning hours may advance the circadian rhythm (Neil‐Sztramko et al. 2014). Night workers are normally exposed to artificial light during their shifts, yet large individual differences in circadian phase shifting in response to night work are reported as some workers phase delay while others phase advance their circadian rhythm.

Circadian adaptation to the work schedule is usually observed in only a minority of night shift workers (Folkard, 2008). Lack of circadian adaptation results in increased sleepiness and decreased alertness on duty as well as shortened daytime sleep following the night shift. This raises important questions about the role of light exposure in the workers’ adaptation to night shift.

In this issue of The Journal of Physiology, Stone et al. (2018) present results from a field study investigating the role of individual light exposure in adaptation to night shift work among nursing and medical staff in an intensive care unit. Outcome measures comprised individual light exposure and shift in circadian phase measured by the rhythm of the urinary melatonin metabolite aMT6s. The circadian phase was collected at baseline (the last rostered day shift), on their first night shift and on the final consecutive night shift (3 or 4 night shifts). The person's diurnal preference was also measured by the use of the Morningness‐Eveningness Composite Questionnaire (MEQ). Strong individual differences emerged in both the direction (advance and delay) and magnitude of circadian phase shift after 3 or 4 consecutive night shifts. Regression analysis revealed that the night shift worker's ability to phase shift could largely be explained (71%) by the difference in the amount of light exposure relative to the person's circadian phase and his/her diurnal preference measured by MEQ.

Gathering data from night shift workers in a real setting takes time, and complicated project designs tend to lose participants. At the same time, scientists are wary of oversimplifying. One of the biggest challenges is to convey information that is multi‐dimensional and complex in a way that could be accessible.

The study by Stone et al. strongly suggests that future studies should take the light exposure and knowledge of the person's circadian phase into account to improve personalized intervention strategies for shift work. As the authors point out, there is an ‘apparent sex difference’ as none of the six males included in the analysis advanced their circadian phase. Recently, two studies examining light sensitivity, circadian regulation and cognitive performance have shown sex differences (Santhi et al. 2016; Chellappa et al. 2017). Sex differences in light sensitivity might play a key role for individual shift work interventions.

An intriguing approach to optimize light interventions would be to utilize active manipulations of an individual's light exposure. Bright light in the evening/early night delays the circadian rhythm and it causes an immediate, increased alertness. Of note, in terms of melatonin suppression and circadian rhythm entrainment, the ipRGCs are especially sensitive to light consisting predominantly of short wavelengths (∼480 nm). Consequently, an investigation of the effects of increasing the light intensity (in lux) and/or colour temperature during night shift work would be in line with such an approach. Light sensitivity also varies with age, chronotype and eye colour. Thus, moderating the effects of these variables on the effects of light exposure in adaptation to night shift work should also receive attention in future studies.

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Competing interests

None declared.

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Both authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.