At any second, half of our planet is bathed in sunlight and the other half is soaked in darkness. And as Earth rotates, days and nights alternate, and this light-dark cycle has shaped Earth’s life for at least 3.7 billion years. Various organisms have evolved a system, a ‘biological clock,’ to predict the time of day and optimize their behaviors accordingly.
In humans, the biological clock drives our body to run in a roughly 24-hour rhythm. It awakes us when the day begins and lets sleepiness creep in as night approaches. But, pay attention to the word “roughly” here: the period of this rhythm is not exactly 24 hours. In humans, it’s slightly longer. A prominent study1 suggested that in healthy individuals, this intrinsic cycle of activities repeats, on average, every 24.18 hours.
This tiny difference explains why this rhythm is called ‘circadian rhythm,’ which means about-a-day rhythm. Like a slightly inaccurate watch, our internal, biological clockwork needs to be reset regularly to synchronize with the external 24-hour day. Researchers use the term zeitgeber (‘time giver’) to refer to any environmental cue that can reset, or entrain, our circadian rhythm.
And among a list of different zeitgebers, light appears to be the strongest and most important in changing the time of our circadian rhythm.
From the Sun to the Cells
Under the daytime sky, we not only see colors and shapes of things but also capture a time hint hidden in sunlight. A specific type of neuron in the retina sends the light information to a brain region situated above where the optic nerves cross. That small region, known as the suprachiasmatic nucleus, or SCN for short, is the control room of our circadian rhythms. The incoming light induces the expression of particular genes in SCN cells. Then, those cells give rise to cascades of activities that synchronize the individual biological clocks in other cells throughout our body.
The SCN coordinates biological clocks from received light. Credit: NIGMS
In this sense, the SCN serves as one master circadian clock. It controls the circadian rise and fall of critical physiological activities such as body temperature and hormone production. In individuals exposed to light intense enough, for example, the secretion of melatonin, which is often referred to as the sleep hormone, is inhibited2. However, as the environment gets dark, the SCN signals the brain to prepare the body for rest and sleep. Levels of melatonin go up, and you begin to feel sleepiness.
Shine Light at the Right Time
Before electric light became a thing, sunlight was the definitive source of light that drew the line between daytime and evening hours. But now, the boundary of day and night has been blurred by bright artificial lighting.
In his streaming TV series Mind Field, American educator Michael Stevens once explored the impact of isolation by putting himself into a small room for three days3. Way before loneliness and boredom harmed his cognition, bright lights that never turned off in his room disrupted his circadian rhythm. After his first sleep, his sense of time started falling apart. Halfway through, Michael felt it was time to have dinner when it was 11:30 a.m. and believed around 70 hours had passed when he had only stayed in his room for 40 hours.
But our biological clock doesn’t get confused only under such extreme circumstances. Studies have shown that exposure to even dim light at night can provoke a circadian response, even when we keep our eyelids shut4. To keep our circadian rhythm and every related aspect of our physiology on track, we need to carefully monitor and manipulate our daily light exposure.
The key point here is to get light at the right time.
On the International Space Station, which is 400 kilometers above Earth’s surface, astronauts have to rely on delicate lighting systems to maintain their circadian rhythm with the lack of typical diurnal cycles of sunlight.
Here on Earth, we just follow the guidance of the Sun: when it’s out there in the morning, try to get as much sunlight as you can. Have an outdoor breakfast, sit by a sunny window during your break, exercise outside, etc. Once the Sun is gone, keep an eye on how bright your electric lights are. Dim them a few hours before going to bed to minimize light exposure. Dark sunglasses and sleep masks will come in handy when it’s hard to avoid bright light at night.
A Beam of Owaves
OK, I know what you want to say. Controlling one’s light exposure is much easier said than done. While it may not have been a tough task two centuries ago when most humans led a mostly outdoor life, embracing sunlight during daytime and escaping from over-bright night lights requires more effort from urban dwellers nowadays.
A better mindset is to think about light exposure monitoring and manipulation not as an assignment but as a routine. And Owaves facilitates just that. The app allows you to plan your day in detail, narrowing time frames down to every 15 minutes. After syncing Owaves with Apple Health, you can monitor the actual time spent on activities such as sleep and exercise. By actually planning things into your daily routine such as doing outdoor activities and dimming indoor lights, you can gradually train yourself to be more aware of your light exposure and its impact on your health.
1. Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N., Mitchell, J. F., Rimmer, D. W., … & Kronauer, R. E. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177-2181.
2. Duffy, J. F., & Czeisler, C. A. (2009). Effect of light on human circadian physiology. Sleep Medicine Clinics, 4(2), 165-177.
3. Stevens, M., Lieber, K., & Roper, J. [Vsauce]. (2017). Isolation – Mind Field (Ep 1) [Video]. YouTube Originals. https://www.youtube.com/watch?v=iqKdEhx-dD4
4. Tähkämö, L., Partonen, T., & Pesonen, A. K. (2019). Systematic review of light exposure impact on human circadian rhythm. Chronobiology international, 36(2), 151-170.
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