What did randomized controlled trials of phototherapy—bright morning light—for weight loss find? 

If weakening our circadian rhythm can cause weight gain, might strengthening it facilitate weight loss? You may remember the child’s swing analogy I shared previously. Regular morning meals can give our cycles a little daily push, but the biggest shove comes from our exposure to bright morning light. Similarly, exposure to light at night could be analogous to nighttime eating, as you can see below and at 0:31 in my video Shedding Light on Shedding Weight.

Of course, we’ve had candles to illuminate our nights for 5,000 years, but flames from candles, campfires, and oil lamps are “strongly skewed towards the red end of the [light] spectrum; as a result, firelight has much less impact on circadian rhythmicity than electric light.” It’s the shorter blue wavelengths that specially set our circadian clocks. Electric lighting, which we’ve only had for a little over a century, “has gradually changed since the 1960s from an incandescent-bulb form consisting of mainly low-level yellow wavelengths to high-intensity discharge forms,” such as fluorescents and LED lights, “that contain blue wavelengths,” which are more similar to morning sunlight and have the strongest effect on our circadian rhythm.

Using wrist meters to measure ambient light exposure, researchers found that increased exposure to light in the evening and nighttime correlated with a subsequent increased risk of developing obesity over time. This was presumed to be due to circadian misalignment, but might it instead be a sign of not sleeping as much, and maybe that’s the real reason people grew heavier? This was controlled for in a study of more than 100,000 women, which found that the odds of obesity trended with higher nighttime light exposure independent of sleep duration.

Compared to women who reported their bedrooms at night were either too dark to see their hand in front of their face or at least dark enough that they couldn’t see across the room, those who reported their bedrooms were light enough to see across the room were significantly heavier. They weren’t all sleeping with nightlights on either. Without blackout curtains on windows, many neighborhoods may be bright enough to cause circadian disruption. Using satellite imagery, scientists have even been able to correlate higher obesity rates with brighter communities. There’s so much light at night these days that, outside of a blackout, the only Milky Way our children will likely ever see is inside a candy wrapper.

Although sleep quantity could be controlled, what about sleep quality? Maybe people sleeping in bedrooms that aren’t as dim don’t sleep as soundly, leaving them too tired to exercise the next day, for example. You can’t know for sure if nocturnal light exposure is harmful in and of itself until you put it to the test. When that was done, those randomized to exposure to bright light for a few hours in the evenings or exposed even just for a single night suffered adverse metabolic consequences. 

The more intriguing question then becomes: Can circadian syncing with morning bright light therapy be a viable weight-loss strategy? Insufficient morning light may be the circadian equivalent of skipping breakfast. Indoor lighting is too bright at night, but it may be too dim during the day to robustly boost our daily rhythm. Light exposure from getting outdoors in the morning, even on an overcast day, is correlated to lower body weight compared to typical office lighting, so some doctors started trying “phototherapy” to treat obesity. The first case reports began being published in the 1990s. Three out of four women lost an average of about four pounds over six weeks of morning bright light exposure, but there was no control group to confirm the effect. 

Ten years later, the first randomized controlled trial was published. Overweight individuals were randomized to an exercise intervention with or without an hour a day of bright morning light. Compared to normal indoor lighting, the bright light group lost more body fat, but it’s possible the light just stimulated them to exercise harder. Studies show that exposure to bright light, even the day prior to exercise, may boost performance. In a handgrip endurance test, exposure to hours of bright light increased the number of contractions until exhaustion from about 770 to 860 the next day. While light-induced improvements in activity or mood can be helpful in their own right, it would be years later still before we finally learned whether the light exposure itself could boost weight loss. 

Following an unpublished study in Norway purporting to show a dozen-pound weight-loss advantage to eight weeks of 30 minutes of daily daylight (compared to indoor lighting), researchers tried three weeks of 45 minutes of morning bright light compared to the same time sitting in front of an “ion generator” that appeared to turn on but was secretly deactivated. As you can see in the graph below and at 5:08 in my video, the three weeks of light beat out the placebo, but the average difference in body fat reduction was only about a pound. This slight edge didn’t seem to correlate with mood changes, but bright light alone can stimulate serotonin production in the human brain and cause the release of adrenaline-type hormones, both of which could benefit body fat aside from any circadian effects. 

Regardless of the mechanism, bright morning daylight exposure could present a novel weight-loss strategy straight out of the clear blue sky.  

I have a whole series on chronobiology. You can see all of the videos on the topic page. The last few are listed below in the related posts and help to paint the full picture of how our environment can affect our circadian rhythms.

For more on weight loss, you can also check out my recent series in the related posts below, or browse all of my weight loss videos here.