People exposed to morning sunlight are leaner than those who get afternoon light, a new study has found.
The study by Northwestern University found that the timing, intensity and duration of your light exposure during the day is linked to your weight.
People who had most of their daily 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, the study found.
"The earlier this light exposure occurred during the day, the lower individuals' body mass index," said co-lead author Kathryn Reid, research associate professor of neurology at Northwestern University Feinberg School of Medicine.
"The later the hour of moderately bright light exposure, the higher a person's BMI," Reid said.
The influence of morning light exposure on body weight was independent of an individual's physical activity level, caloric intake, sleep timing, age or season. It accounted for about 20 per cent of a person's BMI.
"Light is the most potent agent to synchronise your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance," said study senior author Phyllis C Zee.
"The message is that you should get more bright light between 8 am and noon," Zee said.
About 20 to 30 minutes of morning light is enough to affect BMI, researchers said.
"If a person doesn't get sufficient light at the appropriate time of day, it could de-synchronise your internal body clock, which is known to alter metabolism and can lead to weight gain," Zee said.
The exact mechanism of how light affects body fat requires further research, she noted.
"Light is a modifiable factor with the potential to be used in weight management programmes," Reid said.
"Just like people are trying to get more sleep to help them lose weight, perhaps manipulating light is another way to lose weight," Reid added.
The study was published in the journal PLOS ONE.