Historically, our internal biological clock – or circadian clock – was predominantly set by fluctuations in light across the solar day. Now, however, things are quite different. Technological advances over the past century, starting with the invention of the lightbulb, have dramatically changed our patterns of daily light and darkness exposure. These technologies include artificial lighting, television, and more recently phones and tablets, and they have had a massive influence on what’s considered a normal daily pattern of human behavior.
Today, our cities and homes can be illuminated all the time, regardless of season or time of day. In addition, Americans on average spend about 90% of their time indoors (Klepeis et al., 2001), and are able to extend periods of light exposure well into the evening with artificial lighting. This light signal past sunset can delay the timing of our circadian rhythm, misaligning our internal body rhythms with the natural fluctuations of the solar day. And because we control our light exposure, our lighting timing can change night by night.
Standard HG Lifestyle Pattern
Relatively recently, advances in wearable sleep monitoring technology have enabled us to study the light exposure patterns of non-industrialized people.
One group of particular interest is the Hadza, who are a hunter-gatherer community that live in the savannah of northern Tanzania. This community, we presume, lives in a manner that is relatively similar to that of our ancient ancestors, and they occupy roughly the same geographic region where it is thought that anatomically modern humans evolved (Campbell et al., 2010). This makes them a uniquely appropriate model for understanding ancestral lifestyle patterns.
The Hadza do not use electricity and are exposed to light through the sun, moon, stars, and fire. They wake before sunrise, at roughly the same time every day. They get the most intense light exposure at around 9 am, before retreating to the shade around midday. After dark, they are exposed to light only in the form of fire and lunar fluctuations. (Yetish et al., 2015)
Light – Another Example of Mismatch
If we assess the modern light exposure pattern and compare it to the ancestral pattern, major differences are obvious. First of all, modern humans spend relatively little time outdoors. In the absence of sunlight, which is far more intense than indoor lighting, we don’t receive that strong signal that tells the brain that it is daytime. And because we can extend our period of light exposure after sunset, we are truncating our exposure to darkness and hormonal signals that are generated by it. Lastly, many of us also sleep with a light on at night (like those who can’t sleep without having the TV on).
Do these differences matter?
Light on Vitamin D
Vitamin D is a pre-hormone that is synthesized in the skin when exposed to ultraviolet light. Contemporary estimates (Ginde et al., 2009) have suggested that perhaps three-quarters of US adults have low vitamin D status. In contrast, members of the Hadza tribe were found to have very high levels of serum 25-hydroxyvitamin D, about 110 nmol/L. (Luxwolda et al., 2012)
Vitamin D is essential for maintenance of bone mineralization. It likely impacts many other biological functions as well, as deficiency of vitamin D has been linked to cancer, autoimmune disease, and atherosclerosis. (Wang et al., 2017)
Light on Circadian Timing
The biological clock prepares our bodies for predictable events, like sleep and energy intake. Misalignment of the biological clock may have serious metabolic consequences.
Mice that are exposed to 8 hours of dim light during the time when they would typically be asleep experience 50% greater weight gain than counterparts living under a normal light-dark cycle. Curiously, they did not consume more food, but rather ate when they would normally be asleep. Metabolic effects of circadian misalignment like this could be an important piece of the puzzle in the contemporary obesity and diabetes epidemics. (Fonken et al., 2010)
Similarly, new research has revealed that humans who are exposed to even a tiny amount of light while they are sleeping are much more likely to develop symptoms of depression than those who sleep in total darkness. (Obayashi et a., 2017)
Light on Brain Health
But it’s not simply a matter of getting too much light at night. Some evidence now suggests that bright light is vital for cognitive performance. Researchers compared rats that spent a significant portion of their daytime exposed to either 1) bright light or 2) dim light, in a range of illumination that is ecologically relevant to humans. Remarkably, the rats exposed to dim light lost about 30% capacity in the hippocampus, a part of the brain that is associated with learning and the consolidation of memories. This translated into poorer cognitive performance when navigating a maze. Once the rodents were switched to bright light during the day, these neurological effects were reversed. (Soler et al., 2018)
For millennia, exposure to light and dark by the human genome was rather consistent. Now, this exposure pattern has changed radically in a very short period of time and represents, yet again, just another example of mismatch. While we are still learning the profound impact environment has on the functioning of our physiology, we really don’t need mountains more data to see the light.
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