How Light Improves Sleep

TL;DR Summary

-            Sleep is foundational to health, productivity, and performance, arguably more important than diet and exercise. If you’re skeptical, I recommend Dr. Matthew Walker’s book Why We Sleep or listen to any of his podcast interviews and I’m sure you’ll be convinced as well.

-            As discussed in the previous article, light drives the sleep-wake circadian rhythm, so light is highly important for improving sleep quality.

-            There are studies looking at lighting interventions increasing light levels during the workday through either daylight or electric light, and they show improved sleep quality.

-            Studies also show that high light level exposure earlier in the day and in the days beforehand reduces the SCN’s sensitivity to light at night. Increasing light exposure during the day increases the resiliency of the SCN to light at night.

-            Improving light exposure (and other tactics) to advance the circadian rhythm can help improve sleep quality and mental health by better aligning natural chronotype behaviors to their workday schedule.

Why is Sleep Important?

Sleep is foundational to health, productivity, and performance.[1,2] Sleep is when the body and mind recuperate, repair, cleanse, and build from the activities of the day. Consider what happens when we don’t get enough sleep. Poor sleep (less than 7-8 hours a night for most people) erodes motivation, mood, cognitive ability, stress management, and discipline. It changes our hormone make up such that we crave junk food and feel more hungry: we eat more calories and feel less satisfied for it. Poor sleep wrecks insulin production, severely weakens the immune system, and blunts our physical strength and endurance. One can go on a diet to lose weight, and poor sleep will undermine it by reducing lean body mass instead of fat[3]. A study found that, statistically, those who regularly get 5-6 hours of sleep are 19% less productive than those who get the normal 7-8 hours[4].

I could elaborate of course, but instead, I urge you to read Dr. Matthew Walker’s book Why We Sleep, or listen to any of the plethora of podcast interviews he’s participated in. From my own research and personal experimentation, I am convinced that sleep is foundational to health, arguably more important than diet and exercise since, without high quality sleep, maintaining the other two is incredibly difficult, if not impossible for most people long-term.

How Light Affects Sleep

As we’ve seen, light exposure throughout the day plays a critical role in the quality of a person’s sleep at night. Exposure to high light levels in the morning and throughout the day combined with darkness at night drive a healthy sleep-wake cycle. As designers and developers of the built environment, creating a lighting environment that ensures consistent high light level exposure daily becomes an environmental tool that can help stabilize people’s circadian rhythms and sleep-wake cycle. Indeed, early studies of lighting interventions, whether through daylight or electric light, to increase light exposure during the day have found improved sleep quality[5–8]. For example, a study looking at daylight[7] found that increasing light levels in the office space with daylight (from average of 40 EML to 316 EML), improved sleep duration on average by 37 minutes. The study looked deeper and found that those who were categorized as “poor sleepers” increased their sleep by 56 minutes. Cognition scores, based on a test simulating cognitive demands for knowledge workers, increased by 42% on average, but more to the point, every participant except one experienced an increase in their cognitive score. Another study using electric light[5] found that increasing morning light exposure did not affect sleep duration but did improve sleep quality, mood and circadian entrainment; while increasing light exposure for the whole work day had even better effects and lower depression scores.

These interventions are only looking at improving light exposure during the day while employees are at their office. There is no intervention on the lighting at home, or what people do outside of work. Simply improving light exposure during the day has a ripple effect into the evening when people have left the office, and their sleep is improved. How can this be?

SCN's Response to Light Changes, Depending On Previous Light Exposure

That light powerfully entrains our circadian rhythm helps explain some of this, but there’s something else going on. It turns out that the SCN’s response to light changes depending on how much light it has received throughout the day, and even in the days prior. Several studies have looked at what happens when we expose people to high light levels during the afternoon and early evening then again to light at night when melatonin should be produced[i]. Amazingly, they have found that earlier high light level exposure reduces the melatonin suppression resulting from light at night compared to if they had no additional light exposure at all[9–11]. Another study showed that bright light exposure over the course of a week reduced the impact of light at night, so our light exposure from previous days appears to play a role in the sensitivity of the SCN to light at night[12].

In other words, the history of light exposure from earlier in the day and the days prior changes how the SCN responds to light at night. Increasing daytime light exposure makes people more resilient against lifestyle habits, light pollution, screen time, etc. that would hamper their sleep and erode the strength of their circadian rhythm[ii].

The inverse is also true: reduced light exposure during the day increases the SCN’s sensitivity and response to light at night. Spending the day in biologically murky light levels does not provide sufficient stimulus to adequately saturate the circadian system, leaving people more susceptible to circadian and sleep disruption from light at night, whether from our electronics, home lighting, or light pollution. Lighting today and historically is designed to meet the needs of our visual system, but as I’ve discussed previously, our melanopic system needs more light for longer durations. We live and work in biologically murky lighting environments, eroding our circadian system’s health, sleep quality, and making us more susceptible to light at night.

This weakening of our circadian system, compounded by lifestyle choices, creates the conditions to phase delay our circadian rhythm. In effect, it skews our circadian rhythm towards evening-centric chronotypes[iii] that are out of sync with our workday schedules. We live in a society that generally still expects people to report to work in the morning, regardless of your chronotype, which punishes “night owls” in particular by truncating their sleep. But, I think, it's not just night owls who suffer, but all of us. The biologically murky lighting environments we spend 90% of our time in shifts our circadian rhythms back. It effectively shifts all of us towards more “night owlishness” than we would be under natural lighting conditions.

Aligning Circadian Rhythm and Natural Chronotype Behavior with Light

There is a famous study (as famous as any study researching light and health can be...) looking at what happens when you take a group of people camping[13]. The researchers found that moving people out of the biologically murky lighting environment of the city into a natural lighting environment, on average, shifted their sleep and melatonin cycle (e.g. their circadian rhythm) forward about 2 hours. Most striking is the magnitude of change to the habitual night owls. While those with earlier chronotypes in the city seemed to have hardly any change in their sleep and melatonin onset times while camping, the late chronotypes in the city had a significant shift forward in circadian phase and became nearly aligned with their earlier chronotype counterparts. The difference flattened out, and those who would be considered night owls were, in fact, not nearly as night owlish as they thought when living under natural light conditions.

Another study looked at interventions to help shift habitual night owls to an earlier circadian rhythm using multiple strategies like timing of eating, exercise, caffeine intake, consistent sleep schedule, and, of course, light exposure via morning daylight and reduced evening light[14]. They found that using these interventions shifted their circadian rhythm forward about two hours earlier, including sleep/wake times, and improved their alertness, productivity, and reduced their symptoms of depression. The lighting intervention studies I refer to above didn’t specifically seek to phase advance night owls, but the effect is similar: advancing their circadian system helps to better align people’s natural chronotype to the day people actually live, improving how they feel. Anecdotally, this aligns with the experience of my wife and I after implementing similar strategies, in particular rigor around bright light during the day (including supplementing with table lamps) and darkness at night.

Light is an environmental tool to help improve people’s sleep quality and anchor their circadian rhythm to better align our natural chronotype to our workday. Better sleep has a ripple effect throughout our lives, resulting in better physical and mental health which can manifest as improved productivity, performance, and sense of vitality and wellness. But the benefits of improving light for our physiology don’t stop there. Light directly affects mental health and cognition through completely separate pathways in the brain than the circadian system and sleep. I’ll talk about that in the next article.

This article is proudly researched and written by myself without the use of AI.

[i] Measuring melatonin production is a common way that researchers gauge the effect of light on the SCN. By knowing a person’s baseline melatonin production under controlled lighting conditions, researchers can expose people to different light exposures at night, such as different light intensities, durations, or spectrums, and measure how much less melatonin is produced. This becomes a proxy for gauging how much of an impact on the circadian system that light exposure has. Measuring the change in melatonin onset, mid-point, and offset is also a method for tracking changes circadian rhythm timing.

[ii] To be clear: it doesn’t eliminate the effect of light exposure at night. Melatonin production is still suppressed when exposed to light at night. But the amount and severity are reduced, adding some resiliency to our circadian system’s health.

[iii] Chronotype is a person’s natural inclination to be active. “Morning larks” get up early and go to bed late and “night owls” are the opposite. About 15% of the population are larks and 15% owls, leaving the rest of use in the “normal” bucket.

List of Citations

1.               Walker MP. Why We Sleep: Unlocking the Power of Sleep and Dreams. First Scribner hardcover edition. Scribner, an imprint of Simon & Schuster, Inc; 2017.

2.               Papatriantafyllou E, Efthymiou D, Zoumbaneas E, Popescu CA, Vassilopoulou E. Sleep Deprivation: Effects on Weight Loss and Weight Loss Maintenance. Nutrients. 2022;14(8):1549. doi:10.3390/nu14081549

3.               Nedeltcheva AV, Kilkus JM, Imperial J, Schoeller DA, Penev PD. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann Intern Med. 2010;153(7):435-441. doi:10.1059/0003-4819-153-7-201010050-00006

4.               Yang R, Hale L, Branas C, et al. 0189 Work Productivity Loss Associated with Sleep Duration, Insomnia Severity, Sleepiness, and Snoring. Sleep. 2018;41:A74-A74. doi:10.1093/sleep/zsy061.188

5.               Figueiro MG, Steverson B, Heerwagen J, et al. The impact of daytime light exposures on sleep and mood in office workers. Sleep Health: Journal of the National Sleep Foundation. 2017;3(3):204-215. doi:10.1016/j.sleh.2017.03.005

6.               Boubekri M, Cheung I, Reid K, Wang CH, Zee P. Impact of Windows and Daylight Exposure on Overall Health and Sleep Quality of Office Workers: A Case-Control Pilot Study. Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine. 2014;10:603-611. doi:10.5664/jcsm.3780

7.               Boubekri M, Lee J, MacNaughton P, et al. The Impact of Optimized Daylight and Views on the Sleep Duration and Cognitive Performance of Office Workers. International Journal of Environmental Research and Public Health. 2020;17(9):3219. doi:10.3390/ijerph17093219

8.               Figueiro MG, Plitnick BA, Lok A, et al. Tailored lighting intervention improves measures of sleep, depression, and agitation in persons with Alzheimer’s disease and related dementia living in long-term care  facilities. Clin Interv Aging. 2014;9:1527-1537. doi:10.2147/CIA.S68557

9.               Chang AM, Scheer FAJL, Czeisler CA. The human circadian system adapts to prior photic history. J Physiol. 2011;589(Pt 5):1095-1102. doi:10.1113/jphysiol.2010.201194

10.            te Kulve M, Schlangen LJM, van Marken Lichtenbelt WD. Early evening light mitigates sleep compromising physiological and alerting responses to subsequent late evening light. Sci Rep. 2019;9(1):16064. doi:10.1038/s41598-019-52352-w

11.            Kozaki T, Kubokawa A, Taketomi R, Hatae K. Effects of day-time exposure to different light intensities on light-induced melatonin suppression at night. Journal of Physiological Anthropology. 2015;34(1):27. doi:10.1186/s40101-015-0067-1

12.            Hébert M, Martin SK, Lee C, Eastman CI. The effects of prior light history on the suppression of melatonin by light in humans. J Pineal Res. 2002;33(4):198-203.

13.            Wright KP, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED. Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle. Curr Biol. 2013;23(16):1554-1558. doi:10.1016/j.cub.2013.06.039

14.            Facer-Childs ER, Middleton B, Skene DJ, Bagshaw AP. Resetting the late timing of ‘night owls’ has a positive impact on mental health and performance. Sleep Medicine. 2019;60:236-247. doi:10.1016/j.sleep.2019.05.001