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.