"Shedding Light on Shedding Weight"
If weakening 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, camp fires, and oil lamps are skewed
towards the red end of the light spectrum,
and it's 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 night time exposure correlated
with a subsequent increased risk of developing obesity over time.
This was presumed to be due to circadian misalignment, but
maybe 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 of obesity trended with higher
night time 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.
It's 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's 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 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 even 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 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 overcast days,
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 it's 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.