Light exposure, obesity, and health


Humans exhibit a strong circadian rhythm in many physiological processes, with certain of these processes more strongly at work during daytime hours rather than night, and for others, vice versa. Most obviously, we sleep at night, and at that time melatonin production increases and the metabolic rate decreases, reversing polarities during the day. If this rhythm becomes dysfunctional, health can suffer, as can lean body weight. Working graveyard shifts is associated with a substantially increased rate of cancer and heart disease.

I wrote about using light as one step of many in overcoming chronic fatigue.

The topic of circadian rhythms and health is sufficiently huge that we’ll just focus on a couple of aspects here: obesity and mental health.


A number of studies have looked at the relation between obesity and light exposure. In mice, disruption of the circadian rhythm through exposure to light leads to obesity. In humans, an association has been found between obesity and the amount of exposure to “light at night”. And a recent paper found that Timing and Intensity of Light Correlate with Body Weight in Adults.

In this study, the researchers used a measure of both intensity of light exposure and its time of day, which they called the MLiT500.

The results of this study demonstrate that the timing of even moderate intensity light exposure is independently associated with BMI. Specifically, having a majority of the average daily light exposure above 500 lux (MLiT500) earlier in the day was associated with a lower BMI. In practical terms, for every hour later of MLiT500 in the day, there was a 1.28 unit increase in BMI. The complete regression model (MLiT500, age, gender, season, activity level, sleep duration and sleep midpoint) accounted for 34.7% of the variance in BMI. Of the variables we explored, MLiT500 contributed the largest portion of the variance (20%).

According to this, the amount and degree of exposure to light explained more of the variance in obesity than did age or even activity level, that is, physical exercise.

Daily light exposure of greater than 500 lux, especially earlier in the day, was important to maintaining a lean body weight. The average indoor room light, the authors explain, is only 100 to 500 lux, so the intensity of exposure needed is that equivalent to outdoors. Outdoor light, even on a cloudy day, can be many times more intense than indoor lighting, a fact that goes unappreciated by many, since our eyes adjust to the amount of light available.

Is the association between light exposure and obesity causal in the direction indicated, or mere association? While that question can’t be definitively answered, it certainly appears causal, and the study references a number of other studies that show how sleep duration and light exposure disrupt glucose metabolism, appetite, and body fat. It’s also been shown how short sleep duration causes disruption in the levels of the hormones leptin and ghrelin, which are responsible for regulating hunger. And as mentioned above, animal studies show that light exposure at the wrong times can cause weight gain. So all this points in the direction of causality. It also seems possible that the amount of light exposure indicated is a mere marker for sleep, since if one sleeps late into the day, then exposure to bright light in the morning will be less. This idea makes sense in that it will also explain the known association between short, poor sleep and obesity.

If you are trying to lose weight, it would seem important to get exposure to bright light early in the day. Getting up early and walking or exercising outside could have real health benefits, especially if you work indoors and won’t get much other exposure during the day. A suggestion from Dr. Daniel Kripke, a psychiatrist who has extensively studied the health effects of sleep, is to eschew the use of sunglasses when driving to work in the morning, which may allow enough marginal bright light exposure to make a difference. (I highly recommend Dr. Kripke’s e-book, free at the link.) In winter, one can use a bright light box; the linked one is rated at 10,000 lux, and I used this version when I was trying to overcome chronic fatigue, at which I was successful. (How much light played a role I’ll never know, although I did shift work most of my adult life.)

Since late sleep and early light exposure are negatively related, and since poor sleep is also associated with obesity, optimizing sleep is important, and here light also plays a role. Exposure to light in the blue end of the spectrum seriously disrupts the production of melatonin, the sleep hormone. Blue light comes predominantly from video and computer monitors, so if you use a computer, including a tablet, at night, you should have the programs f.lux (for PC) or Twilight (for Android) installed; these automatically change the color temperature and intensity of the screen at the appropriate times for your latitude. I’ve had good success with them, my sleep being noticeably improved since I installed them. Blue-blocking glasses also work in this way, but you have to remember to wear them, which I consistently failed at doing, so I like the computer programs better.

Light exposure plays only one role in weight gain, though whether it’s a large or small role seems difficult to say at this point. However, as I emphasized in my book on obesity, we live in what has been characterized as an obesogenic environment, in other words, an environment in which many factors figuratively conspire to make us overweight. Eliminating the various factors one by one is a good strategy for weight loss, and light exposure, in the appropriate amount and at the right times, and not at the wrong times, is one of those factors that should be fine-tuned.


Sleep and depression are closely related. On the one hand, insomnia is characteristic of depression, but on the other, one night of total or partial sleep deprivation causes instant relief of depression in most patients.

As for light, it’s been found that light therapy is not only effective against depression, but that the effect size is as large as that of antidepressant drugs. Here’s a treatment that the drug companies don’t want you to hear about. In their defense, though, most people would rather pop a pill than make any effort to help themselves, that is, if it takes real effort.

The type and length of light therapies have not been standardized, but typically light therapy is begun first thing in the morning, using a light box of 10,000 lux brightness, duration of one-half to one hour. Even one session has been shown to have effects on depression. Side effects are few; those with a tendency to mania may want to avoid light therapy, as that can be an effect in those so inclined. (We also see the same thing in sleep deprivation therapy.)

For those not clinically depressed, a simple walk in morning sunshine should help keep mood elevated. In sleep deprivation, mood becomes elevated even in those not clinically depressed, so we could hazard a guess that light therapy should do the same.


Leave a Comment:

Remnant says February 15, 2015

I don’t know if there is clinical / empirical evidence of the practice, but in order to mimic the natural rhythms, I take my Vitamin D supplement in the morning for the same reason. Beyond a certain hour, if I haven’t taken my Vitamin D, I’ll skip it until the next day.

    P. D. Mangan says February 15, 2015

    I remember Seth Roberts discussing that. I do the same.

      Joshua says February 18, 2015

      I believe the idea of taking Vitamin D in the morning was also brought up in the Perfect Health Diet. I follow the idea too.

BigFatGuy says February 15, 2015

That is really interesting. The weather in Florida is pretty nice this time of year, and there is nothing more glorious than riding a bicycle under a bright sun in a clear blue sky. I ride the bike trails around here until my legs fall off. Sometimes I cannot get to sleep at night because I hurt so much. It’s wonderful. I can see how some people (not me) could become obsessed with bicycling to the point of spending thousands of dollars on a fancy carbon fiber bike. I have not been riding in the morning particularly, but will alter my schedule in light of this new information.

By the way, I started bicycling quite by accident. My car broke down, and I took it to the shop for repair as usual. In the meantime, I had to get around by bicycle. I thought, hey, this is Florida — I can do this year round, so I sold the car and haven’t driven in a couple years.

Allow me one more personal anecdote. I suffered from insomnia as I got older. Getting a good night’s sleep was difficult for me. I put it down to being like my dad who also suffered from it. Then I started taking Magnesium, and slowly, over time, the insomnia went away. I guess I had some sort of nutrient deficiency. I just wish dad was still around. It might have helped him too. He was addicted to prescription sleeping pills, but still complained bitterly about not being able to sleep.

I suppose people really do get sick sometimes, and doctors have their place, but exercise and proper nutrition can go a long way toward curing most ailments.

I am allegedly 63, but I sure don’t feel 63.

Ben says February 16, 2015

If you are using an rooted android device you should try cf.lumen instead of twilight – where twilight overlays everything with red, cf.lumen only removes the blues, leaving black colours intact.. Looks neater imho

Eric Astrauskas says April 4, 2015

This makes sense when you think about hibernation. Bears go into a dark cave and sleep for months. Their body has to slow down and burn fat at a much lower rate. Otherwise, their bodies would break down to fast and they would die. It is a step to compare bears to humans but possibly the same idea applies. With lack of sun/light = fat doesn’t metabolize as fast.

Soap, Shampoo, and Cold Showers - Rogue Health and Fitness says February 1, 2017

[…] ancestors did not enjoy, and which might possibly have kept them healthier. For example, exposure to light and how it affects sleep and other aspects of health has come to attention; along with eating whole, unprocessed food and […]

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