Morning sunshine does more than get you up - it can improve your weight, research suggests.
Scientists found that light exposure early in the day is linked to lower body mass index (BMI).
Previous research suggests that light may play a role in regulating metabolism, hunger, and the sensation of "feeling full" after eating.
"The message is that you should get more bright light between 8am and noon," said US study author Professor Phyllis Zee, from Northwestern University.
As little as 20 to 30 minutes of morning light is thought to be enough to affect BMI, a measurement of weight related to height.
A BMI of more than 25 is said to classify a person as "overweight" while 30 and over is defined as "obese".
Wrist monitors were used to test the light exposure of 26 male and 28 female study participants for a week in normal living conditions.
"The earlier this light exposure occurred during the day, the lower individuals' body mass index," said co-author Dr Kathryn Reid, from Northwestern's Feinberg School of Medicine.
"The later the hour of moderately bright light exposure, the higher a person's BMI."
The influence of morning light on weight was independent of an individual's activity level, calorie intake, sleep timing, age or season.
It accounted for about a fifth of a person's BMI, according to the scientists writing in the online journal Public Library of Science ONE.
Many people do not get enough natural light in the morning because of their indoor lifestyle, the researchers claim.
The study found that a light level of 500 lux was the "magic number" for having a lower BMI.
Even on a cloudy day, outdoor light produces more than 1,000 lux of brightness. In contrast, indoor environments often have no more than 200 to 300 lux of light.
"Light is a modifiable factor with the potential to be used in weight management programmes," said Dr Reid. "Just like people are trying to get more sleep to help them lose weight, perhaps manipulating light is another way to lose weight."
Prof Zee said insufficient light at the right time of day may de-synchronise the body's internal clock. This in turn could alter metabolism and lead to weight gain.