A surprising new strategy for managing your weight? Bright morning light.


A new Northwestern Medicine® study reports the timing, intensity, and duration of your light exposure during the day is linked to your weight — the first time this has been shown.


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’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 exposure on body weight was independent of an individual’s physical activity level, caloric intake, sleep timing, age, and the season of the year. It accounted for about 20 percent of a person’s BMI. 


“Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance,” said study senior author Phyllis C. Zee, M.D. “The message is that you should get more bright light between 8 a.m. and noon.” About 20 to 30 minutes of morning light is enough to affect BMI.


Zee is the Benjamin and Virginia T. Boshes Professor of Neurology and director of the Northwestern Medicine Sleep and Circadian Rhythms Research Program at Feinberg. She also is a neurologist at Northwestern Memorial Hospital.


“If a person doesn’t get sufficient light at the appropriate time of day, it could de-synchronize 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. 


The study was published April 2 in the journal PLOS ONE. Giovanni Santostasi, a research fellow in neurology at Feinberg, also is a co-lead author.


Visit the Northwestern News Center for the full story.