Does Light Exposure Affect Hunger?

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As you know, at humanOS we emphasize the importance and interrelationships between diet, physical activity, and sleep. For simplicity, I often wrap circadian rhythms under the sleep umbrella. Circadian rhythms are 24 hour patters of biological activity that – under normal environmental conditions – align themselves with the light and dark cycle of one full day. While sleep and wake activity are fundamentally connected to this process, most if not all biological process are under the influence of this cyclical pattern. Light is the most significant moderator of the master rhythm and it also directly stimulates physiological processes, like cognition. For example, light not only helps your brain know when to be awake during the day but the perception of light by the brain directly stimulates brain activity (acts like a cup of coffee, if you will). This has implications for processes regulated by the brain, like appetite, raising an interesting question: does light exposure affect hunger?

 

Does light exposure affect hunger?

Chronic sleep deficiency is becoming endemic to modern society and there is great interest now in the sleep research community to understand how sleep restriction affects our health. One line of research has looked at how sleep restriction influences hormones that affect hunger, including leptin which can be thought of as a fullness signal. It has now been shown several times that sleep restriction can influence these hormones so that you’re hungrier during the day. However, most people who undergo chronic sleep curtailment (voluntary sleep restriction due to something like staying up later to watch more TV) are not staying up in complete darkness. Rather, they are in an artificial light environment. Not only are people losing out on sleep but they are also increasing the fraction of time in a 24 hour period that they are exposed to light and decreasing the fraction of time they are exposed to darkness. It is likely that some of the negative health effects we associate with sleep loss are moderated by increased light and decreased darkness.

New research from Figueiro et al.,  – to be presented next week at the European Sleep Research Society in Paris – looked at how different colors could counteract the impact of sleep restriction on biomarkers associated with hunger (i..e, leptin). In their 9 week study (n=11), the researchers first sleep restricted subjects to 5 hours per night for a week. When the subjects awoke, they were exposed to dim red light for two hours. Next, the subjects still had their sleep restricted to 5 hours a night but when they awoke, they were now exposed to 2 hours of brighter light of one of three different colors: red, green, and blue.

What they found is that sleep restriction significantly reduced leptin levels when compared to these subjects’ normal values. When the subjects had the opportunity to increase their sleep time back to 8 hours, leptin was increased 20%. Most interestingly, however, is that light intensity and spectrum in the morning affected leptin values by over 50%!  There were some differences between the colors in their ability to induce this effect. In fact, the order of significance for these colors from most to least was: red, green, blue, which is actually the opposite of what you might expect given previous research. Perhaps we have different sensitivity to bands of light at different times of day.  Looking at the big picture, this suggests that light can potentially impact obesity by reducing hunger that accompanies sleep deprivation. However, it appears that both sleep and light are both important. humanOS guidance is to not only prioritize a full nights rest in your daily health practice but to also get full-spectrum natural light exposure first thing in the morning.

 

How does light exposure affect hunger? An example of a problematic scenario

It is not uncommon for programmers to stay up late at night. I think this has a lot to do with the fact that this lifestyle is fostered in part by productivity. I’m in Europe right now, so I’m not getting emails every minute during my day. This lower level of distraction is great for productivity! I think programmers and writers experience less interruption by working at night, and this this schedule becomes a habit. The stimulation of artificial light and demanding cognitive tasks of programming / writing masks the homeostatic sleep pressure that would otherwise knock them out cold. Even if they arrive at work later (as many companies allow) I would still guess that chronic sleep restriction is common in this population. Then, when the get to work, they may also be working in a low light environment. Even bright computers screens have far less light intensity than outdoor light (orders of magnitude difference). In fact, brightness is a bit misleading. It is not the same as light intensity. Brightness refers to relative luminocity against a backdrop. Consider an iPad: At night and at maximal brightness, an iPad feels as though it may melt your retinas but if you take the same screen outside during bright daylight, you can hardly see it. So, for this population, we now have sleep deficiency and low daytime light. This new research suggests that this could be a very problematic pattern for weight gain and this pattern is in no way restricted to programmers, I just think it’s common amongst this group.

 

Get a full nights rest and get bright light in the morning and during the day. Time for a break, I’m going to go take a walk outside.

Cheers from Leiden!

Published by Dan

is passionate about food, movement, and sleep. Interested in developing low-cost, high value health solutions. Also interested in anthropology, evolutionary biology, exercise and inactivity physiology, cognition, neuroeconomics, decision making, circadian biology, epistemology, gastronomy, food culture and politics, agriculture, sustainable practices, and dogs. Activities include mountain biking, CrossFit, hiking, dancing, and long walks with my headphones.

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