Can circadian syncing with morning bright light therapy be a viable weight-loss strategy?

Remember the study where earlier eaters steadily lost more weight? To their surprise, the early eaters were eating as much as the late eaters despite the difference in weight loss magnitude. The early eaters ended up about five pounds lighter than the late eaters by the end of the 20-week study, even though they were apparently eating the same amount of food. There didn’t seem to be any difference in physical activity between the two groups either. Could it be that just the timing itself of calorie intake matters? Scientists decided to put it to the test, which well cover next.

If disrupting our circadian rhythm can cause weight gain, might strengthening it facilitate weight loss? In our child’s swing analogy, 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. Yes, we’ve had candles to illuminate our nights for 5,000 years, but flames from candles, campfires, and oil lamps are skewed towards the red end of the light spectrum, and its the shorter blue wavelengths that specially set our circadian clocks. Even incandescent electric lighting starting a little over a century ago consisted of mainly low-level yellow wavelengths, replaced over just the last few decades with fluorescents and LED lights that now contain extra blue wavelengths, which is more similar to morning sunlight, and has the strongest effect on our circadian rhythms.

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

Compared to women who reported their rooms at night were too dark to see their hand in front of their face, or at least too dark to see across the room, those who reported that it was light enough to see across their bedrooms at night were significantly heavier. Its not that they were all sleeping with nightlights on; without blackout curtains on the 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 is so much light at night these days that, outside of a blackout, the only Milky Way our children will likely ever see is in a candy wrapper.

Although sleep quantity could be controlled for, what about sleep quality? Maybe people sleeping in less dim bedrooms don’t sleep as soundly, leaving them too tired to exercise the next day or something. You cant 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 even a single night, suffered adverse metabolic consequences.

Insufficient morning light may be the circadian equivalent of breakfast-skipping. Indoor lighting is too bright at night, but may be too dim in 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 started to be published back in the nineties. 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 its possible the light just stimulated them to exercise harder. Studies show that bright light exposure, 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 8 weeks of 30 minutes of daily daylight compared to indoor lighting, researchers tried three weeks of 45-minute morning bright light, compared to the same time sitting in front of ion generator that appeared to turn on, but was secretly deactivated. 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.