You are when you eat: How the body’s internal clock influences our eating habits

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“It’s not just what you eat, it’s when you eat,” said David Barker, an associate professor of psychiatry and human behavior and author of a Feb. 18 study establishing the relationships between circadian rhythms and caloric intake in adolescents.

Warren Alpert Medical School researchers, in partnership with researchers from Mass General Brigham, found that adolescents who were overweight or obese ate their calories later in their day and circadian cycles.

For participants at a healthy weight, circadian rhythms had a greater influence on caloric intake compared to those who are overweight, said Mary Carskadon, a professor of psychiatry and human behavior and author on the study.

The study found that both behavioral and internal circadian rhythms impact caloric intake, disentangling the impact these two cycles have on human nutrition.

The endogenous — or internal — circadian rhythm is the human body’s 24-hour clock that regulates physiological processes and behavior. This is the first study to provide direct evidence that this internal clock influences a human’s caloric intake, according to the paper.

The behavioral circadian rhythm concerns a person’s sleep-wake cycles and is usually connected to the endogenous circadian rhythm. The endogenous circadian rhythm can persist for a period of time, regardless of light cues, and the behavioral circadian rhythm relies on external factors. 

To conduct the study, researchers tested 51 adolescents from ages 12 to 17. The adolescents were recruited based on body mass index and analyzed in three groups: healthy weight, overweight and obese. 

Participants spent 11 days and 10 nights in the sleep laboratory with no clocks or natural light, Carskadon said. To prepare before for the laboratory, participants spent 14 nights on a regimented sleep schedule at home, wearing an eye mask, keeping a sleep diary and sleeping for a minimum of 10 hours per night.  

Once they began their trial, the adolescents stayed in a room with dim lights without knowledge of the time. Researchers constructed a 28-hour cycle, meaning that for these eleven 24-hour days, participants essentially lived nine 28-hour days. This forced desynchrony protocol “decoupled” their endogenous circadian rhythm from their normal behavioral cycle, Barker said. 

“Through the design, we were able to separate and look at those influences separately from one another, which we can’t do under normal conditions,” he said. 

During these 28-hour cycles, participants ate six meals while living somewhat normally, Barker explained, adding there were crafts and movies.

The researchers collected saliva samples from each participant every half hour while they were awake to assess melatonin levels, a marker for a person’s endogenous circadian rhythm, according to Carskadon.

Barker added that more research should be done before coming to any definitive conclusions from this study. But there is one thing he knows for sure when it comes to the relationship between food and weight: “Timing matters.”