Pssst… wanna lose a few pounds? Get yourself out of bed as early as possible and bathe in the morning light for 20 to 30 minutes. Sure, you’re thinking, another ridiculous weight loss tip! But this one, believe it or not, is backed by science.

A new study from Northwestern University finds that the timing, intensity, and duration of your daily light exposure is linked to your weight. In fact, the researchers discovered that participants who had most of their exposure to even moderately bright light in the morning had a significantly lower body mass index (BMI) than those who had most of their light exposure later in the day. “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,” wrote the authors in the conclusion of their study.

Timing Your Light Exposure

The researchers began by recruiting adult participants through advertisements for a study of circadian rhythms and sleep patterns. Screened for depression, the participants had no major health conditions and none reported doing shift work. Half were women and the average age was about 30. Participants were given seven days of diet logs, sleep logs, and a wrist actigraph, which they were instructed to wear at all times. (A wrist actigraph is a device worn on the wrist to record movements that can be used to estimate sleep parameters.) In their daily diet logs, participants listed the time and location whenever they ate something as well as a description of each food (quantity, preparation, name brand, etc.). In the sleep logs, participants reported both sleep and wake times.

The researchers collected data in a fairly even distribution across all four seasons. Light levels were determined at the wrist using the Actiwatch. For each participant, the researchers computed caloric intake each day and then calculated the mean for the seven-day period. After crunching the numbers, what did the researchers discover?

Participants who received a majority of their daily light exposure above 500 lux earlier in the day had lower BMIs. (Normal room light is typically between 150 to 500 lux, whereas outdoor light is more than 1,000 lux, even on a cloudy day.) "The earlier this light exposure occurred during the day, the lower the individuals' body mass index," said Dr. Kathryn Reid, research associate professor of neurology at the Feinberg School of Medicine and co-lead author. "The later the hour of moderately bright light exposure, the higher a person's BMI."

Significantly, the influence of morning light exposure on body weight was independent of a participant's physical activity level, calorie intake, sleep timing, age, or season. Plus, it accounted for about 20 percent of a person's BMI. “Our results suggest that the relationship between light and BMI is not simply a function of the accumulated minutes of light during the day, but more importantly the temporal pattern of light exposure above a biological threshold,” the authors wrote. Though this study did not investigate why light exposure affects body fat, prior studies have demonstrated that light plays a role in regulating metabolism, hunger, and satiety.

Many people do not get enough natural light in the morning, explained senior author and professor of neurology Dr. Phyllis C. Zee, because the American lifestyle is predominantly indoors. "Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance," Zee stated, further explaining that about 20 to 30 minutes of morning light is enough to affect BMI. "The message is that you should get more bright light between 8 a.m. and noon," she said.

Source: Reid KJ, Santostasi G, Baron KG, Wilson J, Kang J, Zee PC. Timing and Intensity of Light Correlate with Body Weight in Adults. PLOS One. 2014.