Sleep: The Power of Dusk and Dawn

Updated July 31, 2023 | Reviewed by Monica Vilhauer

There are countless articles warning against the disruptive effects of our phones, computers, and television screens on sleep that have led to a frenzy of blue-light glasses, auburn screens and other ways to reduce the light our eyes take in, in the hopes of protecting sleep. However, there is little evidence supporting the claims about light emitted from small screens and there is at least one circadian neuroscientist who is calling the bluff of this frequently repeated assumption and misinterpretation of the data.

Russell Foster explains in his book Life Time (Yale University Press, 2022), in the chapter "The Power of the Eye," that blue lights do stimulate our photoreceptors; however, a light-emitting book is about 31 lux (lux is how light is measured) and someone using it for four hours had barely a ten-minute change in their ability to sleep. Even exposure of 100 lux for several hours has a small impact on sleep and circadian rhythms. One thousand to 5,000 lux may have a bigger impact, but this is a level brighter than most home offices (see diagram above). There is much stronger scientific evidence that sleep is impacted much more significantly by a lack of sufficient light that leads to weakening of our circadian rhythms.

Natural sunlight impacts when we fall asleep and how we feel during our active hours. Sunlight is interconnected with our underlying biological processes and impacts mood, cognitive function, and behavior. Sunlight is the single most powerful way to entrain our sleeping and waking cycles to function optimally. It has perhaps been underestimated for its photoentrainment power, while the influence of other things like cell phone light have been overestimated.

To say it another way, not being able to fall asleep when you want to at night is more likely a deficit of sunlight at dawn than a problem with any other light source. When we have dawn-light exposure (morning sun), it advances our circadian clock and we can fall asleep earlier at night. Without dawn-light exposure, we tend to stay awake later at night, especially when combined with dusk-light exposure in the evening. It may sound strange — even backward — but your morning brain is calculating light in a very serious way.

It gets even stranger than that: The bright midday sun has less power toward strengthening our circadian clocks compared to the light at dawn and dusk. Dawn and dusk are the most powerful times of day to entrain rhythms, possibly because of the way that light is scattered like a prism by particles in the atmosphere, leaving a blue light bubble above us. We don’t know this for sure — the scientific story is still unfolding in this area.

What we do know is that light at dawn advances our clock and we feel sleepy earlier that night. Night owls (late chronotypes) tend to be out and about in the evening time, which may contribute to maintaining their longer than 24 hour days, late nights, and difficulty waking in the morning. Exposure to light at dusk delays the clock in our SCN (suprachiasmatic) clock in our brain. In other words, dusk-light postpones our sleepiness. So, people who miss dawn-light but are out at dusk, may find they don’t fall asleep as early as they want to. People exposed to light at both dawn and dusk may find their circadian rhythm stays strong and balanced.

A study by Wright, et al, (2013) found just that — people who went tent camping for a week slept better and were able to advance their sleep clocks by two hours within that single week of natural light exposure. Since they were exposed to natural light most of the day, they were able to go to bed when tired (earlier than at home) and sleep until dawn-light.

Another interesting piece to this is that photosensitivity and photoentrainment are less related to rods and cones in our eyes as previously thought. Rods are already saturated by 100 lux and cones provide high color contrast vision. More recently, Foster discovered another light sensing group of cells called pRGCs. Due to these specialized photoreceptors, people who are blind but who still have eyes can still use light to entrain their circadian clocks, which drastically improves their quality of life.

Other people who may struggle disproportionately with sleep and circadian rhythm disruption include elderly populations (who may also have declining eyesight), people who are housebound, severely ill or hospitalized, and marginalized people with fewer job schedule choices. If you are a caregiver or boss in charge of other people’s schedules, allowing more schedule flexibility to support light exposure at dusk and dawn could be life-changing for those you care about, leading to improvements in quality of life, health, and work. Please refer to the summary below of how to improve sleep and waking with natural light exposure.

References

Foster, R. (2022). Life Time: Your Body Clock and Its Essential Role in Good Health and Sleep. Yale University Press.

Wright, K. P., Jr, McHill, A. W., Birks, B. R., Griffin, B. R., Rusterholz, T., & Chinoy, E. D. (2013). Entrainment of the human circadian clock to the natural light-dark cycle. Current biology : CB, 23(16), 1554–1558.