How Exposure to Morning Light Affects Your Weight

Written by Lloyd Alter

just-released study determines that, “Light exposure can influence sleep and circadian timing, both of which have been shown to influence weight regulation,” and suggests that people who get a lot of morning light have lower body mass indices. Interestingly, it isn’t just the amount of natural light that people get, but the timing of it. Essentially, early birds are going to tend to be skinnier than night owls. Or as one of the authors of the study says, “The earlier this light exposure occurred during the day, the lower individuals’ body mass index. The later the hour of moderately bright light exposure, the higher a person’s BMI.”

While the Daily Mail totally twists this story into a tale of how sunbathing in the morning helps you to lose weight on holiday, there are some interesting design lessons here.

Getting lots of natural light matters.

Dr. Phyllis Zee, senior author of the study, has some recommendations, quoted in Health Day:

Whenever possible, be exposed to early light. Walk to work if you can. Bright, outdoor light will be way above the 500 lux. If you can’t get outside, work near a window. If you can’t get near a window, at least make sure your work environment is well-lit.

Color temperature matters.

I have written often about how the color of natural light changes through the course of the day, how morning sun is bluer when the sun is lower and has to go through more atmosphere, filtering out the reds. According to the study, this could be significant:

It is also possible that the natural changes in the intensity and wavelength composition of light in the morning compared to the afternoon/evening may in part explain our finding for a differential effect of earlier vs. a later daytime light exposure pattern and BMI. For example, there is generally a higher amount of blue light (shorter wavelength) in the morning. Blue light has been shown to have the strongest effect on the circadian system, including the suppression of nocturnal melatonin secretion.

It’s long been known that natural light changes through the day and that artificial light is a poor substitute; see Thomas Alva Edison Makes You Fat. This has interesting implications for the LED lighting industry too, as new color temperature tuneable bulbs like the Philips Hue become more popular. Who knows, they might now be pitched as a weight loss aid.

Morning sun in the attic. Photo credit: Lloyd Alter/CC BY 2.0

Orientation matters.

For the last few weeks I have been sleeping in a third floor attic room with an east exposure, and being so high, getting lots of morning light. I can honestly say I feel happier in the morning when the light floods in. Perhaps more consideration should be given to designing homes to maximize morning light in bedrooms. As the study concludes:

Light is a powerful biological signal and appropriate timing, intensity and duration of exposure may represent a potentially modifiable risk factor for the prevention and management of obesity in modern societies.

Its another reason to dump Daylight Saving Time

Is DST contributing to our obesity crisis by shifting an hour of sunlight from the morning to the afternoon? Another doctor commenting on the study says:

I do think there’s some evidence here to suggest that the impact of light may influence metabolic changes and possibly play a role in BMI. And, with 67 percent of Americans being overweight or obese, it’s certainly an intervention worth looking at.

Read the whole study here.

This post originally appeared on TreeHugger

Photo credit: Edward Hopper, Public Domain


Borg Drone
Past Member 4 years ago

It doesn't affect my weight.. it affects my eyes. :)

Mary Boorman
Mary Boorman4 years ago

I have come across this research before now and yes, I agree that it is entirely
possible and explains why I have put on another few lbs sleeping in the lower
level apartment at home. It is dim and morning light does not filter through!
Better get moving! And ... walking to work is wonderful!

Camilla Vaga
Camilla Vaga4 years ago


Donna F.
Donna F4 years ago


Julie W.
Julie W4 years ago

The last thing I want in my hot climate is morning sun in the bedroom! OK in cooler climes, but not where I live. I keep morning and afternoon sun off the walls with deciduous trees.

Manuela C.
Manuela C4 years ago


janice b.
jan b4 years ago

This is silly

Teresa W.
Teresa W4 years ago

This is not always true. My grandmother, who is an extremely early bird (for her, getting up later than 5 a.m. is sleeping during the day), has always been very fat. There must be other factors as well.

Shan D.
Shan D4 years ago

"...morning sun is bluer when the sun is lower and has to go through more atmosphere, filtering out the reds." (article)

Uh-huh. That's why the sky in late afternoon is often red (or orange, or pink, or purple). Because the reds have been filtered out due to the Sun being lower and having to go through more atmosphere.


Paul C.
Paul C4 years ago

Interesting article. Where's the hard scientific data to back it up?