Get 15% Off on President's Day Sitewide Sale Get 15% Off on President's Day Sitewide Sale


Exposure To Light Could Promote Lower BMI

Light Exposure

Story at-a-glance -

  • People who had exposure to light in the morning were most likely to have a lower body fat percentage regardless of how many calories they consumed, how much activity levels they had, or how long they slept
  • The benefit was most pronounced in early morning hours. BMI rose by 1.28 points for every hour that light exposure was delayed
  • Past research has shown exposure to light may help reduce body weight, body fat, and appetite, while supporting optimal hormone levels, among obese and overweight individuals
  • Exposing yourself to intense light (aka daylight) for at least half an hour or more each day, and avoiding exposure to light at night (especially in the blue wavelength) is important for optimal health and weight control

By Dr. Mercola

Your ancestors and other life forms developed organs that took advantage of the Earth's environmental cues. We developed a biological clock governed by Earth's cycles of light and darkness.

Even though in the modern day artificial light at night, and dimly lit indoor environments during the day, threaten to throw this carefully orchestrated rhythm completely off balance – with far-reaching effects on your health.

Your circadian rhythm -- the 24-hour cycles known as your internal body clock -- needs bright light to reset itself after a night's sleep. This is why getting 30 minutes of morning sunlight is a habit I urge virtually everyone to get into, as it will send a strong message to your internal clock that day has arrived.

This is important for many reasons, including, as new research has highlighted, maintaining a healthy weight (and even losing weight).

Morning Light May Help You to Shed Pounds

A recent study published by Northwestern University researchers found that people who had exposure to light in the morning were most likely to have a lower body mass index (BMI), regardless of how many calories they consumed, activity levels, or how long they slept.1

The benefit was most pronounced in early morning hours. BMI rose by 1.28 points for every hour that light exposure was delayed. The researchers recommended that people should strive to get light exposure between 8 a.m. and noon, for about 20 to 30 minutes.

As for intensity, BMI benefits were noted at 500 lux, which is the intensity of a brightly lit indoor environment. This is an easy level to reach outdoors, where full daylight measures at more than 10,000 lux, and even on an overcast day the light will be over 1,000 lux. The researchers noted:2

"…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."

BMI Is a Flawed Measure, But Other Research Also Supports Light Exposure for Weight Management

There's no doubt that light exposure, and keeping your internal clock in tune with the natural light-dark cycle, is important for optimal health and weight. However, the featured study used BMI as a measure of weight loss, which is not the best indicator. Ultimately, BMI is seeking to indirectly measure your body fat percentage, but it really doesn't succeed very well.

BMI is arrived at by dividing your weight in kilograms by the square of your height in meters. This fails to differentiate between muscle and fat tissue, and it also doesn't take into account the distribution of body fat on your physical frame (which can greatly impact your risk of chronic diseases, including heart disease).

A better measure of body composition would be waist-to-hip ratio, and it would be interesting to determine how this correlates to morning light exposure. That being said, there is other research to support the researchers' notion that light exposure influences your weight quite significantly:

  • Sleep-deprived subjects who were exposed to light for two hours upon waking had improvements to their disrupted levels of the hormones leptin and ghrelin.3
  • When you are sleep deprived, your body decreases production of leptin, the hormone that tells your brain there is no need for more food. At the same time, it increases levels of ghrelin, a hormone that triggers hunger. Light exposure helped to remedy this hormonal recipe for weight gain.
  • Obese women exposed to bright light for at least 45 minutes early in the morning had reductions in body weight and appetite, leading researchers to conclude that morning bright light treatment may be included in weight-control programs.4
  • Adding bright light treatment to a moderate exercise program helped to significantly reduce body fat among overweight and obese individuals.5
Click here to find out why 5G wireless is NOT harmlessClick here to find out why 5G wireless is NOT harmless

Exposure to Light at Night May Also Wreak Havoc with Your Weight

If exposure to light when your body needs it (i.e. in the morning and during the day) helps to regulate weight, then it would make sense that exposure to light at the wrong times may do the opposite – and this is just what research suggests. In one study, mice that were exposed to dim light during the night gained 50 percent more weight over an eight-week period than mice kept in complete darkness at night.6

They also had increased levels of glucose intolerance, a marker for pre-diabetes. The weight gain occurred even though the mice were fed the same amount of food and had similar activity levels, and the researchers believe the findings may hold true for humans as well.

When you turn on a light at night, you immediately send your brain misinformation about the light-dark cycle. The only thing your brain interprets light to be is day. Believing daytime has arrived, your biological clock instructs your pineal gland to immediately cease its production of the hormone melatonin.

Whether you have the light on for an hour or for just a second, the effect is the same — and your melatonin pump doesn't turn back on when you flip the light back off. Melatonin appears to improve weight control by increasing "beige" fat, which is a heat-generating type of fat that helps your body to burn calories rather than store them. If you're regularly exposed to light at night, you're missing out on this fat-burning potential.

Since humans evolved in the glow of firelight, yellow, orange, and red wavelengths don't suppress melatonin production the way white and blue wavelengths do. In fact, the range of light that inhibits melatonin is fairly narrow — 460 to 480 nm. If you want to protect your melatonin, when the sun goes down you would shift to a low-wattage bulb with yellow, orange or red light. Dr. Russel Reiter suggests using a salt lamp illuminated by a 5-watt bulb in this color range.

Morning light contains more wavelengths of blue light, which are known to have the strongest effect on the circadian system, according to the authors of the featured Northwestern study. This, they believe, may explain why morning light had the strongest association with lower BMI.

On the other hand, televisions, computers, and other electronic devices also emit blue light, which tricks your brain into thinking it's daytime if you use them at night. This is why I recommend avoiding watching TV or using a computer or tablet at least an hour or so before going to bed. An alternative is a free computer program called f.lux (see, which alters the color temperature of your computer screen as the day goes on, pulling out the blue wavelengths as it gets later.

Light Exposure Is Intricately Connected to Your Sleep Patterns

Maintaining a natural rhythm of exposure to daylight, and darkness at night, is an essential component of sleeping well. And getting quality sleep is crucial to maintaining a healthful weight. This is yet another way that light exposure may ultimately make or break your weight-loss goals. The reason why light is important is because it serves as the major synchronizer of something called your master clock. This master clock is a group of cells in your brain called the suprachiasmatic nuclei (SCN).

As a group, these nuclei synchronize to the light-dark cycle of your environment when light enters the eye. You also have other biological clocks throughout your body, and those clocks subsequently synchronize to your master clock. Now with the advent of the light bulb, artificial light, high-definition televisions, and any number of lighted electronic gadgets, we're exposed to a lot more light over a 24-hour period, and a lot less darkness. There are significant consequences to this. Shift workers, for example, are known to have higher rates of obesity and diabetes. But you may not have to be a classic shift worker to increase your risk for obesity and other health conditions. As explained by researcher Dan Pardi:

"There's a new type of shift work, where people go to work in the morning, they come home, they spend a little bit of time with their families, and then on some nights they go back to work on their computers until late at night... This is a pattern that many people in the modern workplace maintain. Again, this new type of shift work significantly alters our internal rhythms because on some nights you're getting light exposure for hours later than the day before. This variability in light instructs your rhythm-setting centers to constantly change body rhythms to try to catch up to the new pattern."

Exposure to Outdoor Light During the Day Is a Key to Optimal Health

Most people in Western societies spend the larger portion of each day indoors, which essentially puts you in a state of "light deficiency." In terms of light intensity, outdoor light is far more intense than indoor light, which is why, if you want to have properly aligned circadian rhythms, strive to maintain natural and proper light rhythms.

This includes exposing yourself to intense light (aka daylight) for at least half an hour or more each day. A gadget that can be helpful in instances when you, for some reason, cannot get outside during the day is a blue-light emitter. Philips makes one called goLITE BLU. (You can find it on Amazon for less than $150.) It's a small light therapy device you can keep on your desk. It's especially useful during the winter when it's harder to get outside (i.e., less daylight hours and less light intensity during daytime). Use it twice a day for about 15 minutes to help you anchor your circadian rhythm if going outdoors is challenging.

Then, in the evening, you want to dim environmental lights and avoid the blue light wavelength. Use blue-blocking light bulbs, dim your lights with dimmer switches and turn off unneeded lights. If using a computer, install blue light-blocking software like f.lux.7 Last but not least, when it's time to go to sleep, make sure your bedroom is very dark, with not even the glow from your clock radio to disturb you. And remember, no clocks with green or blue lights in your bedroom, only amber or red.