Scientists have discovered that circadian rhythms—our internal clocks—play a major role in when and how much we eat.

In a carefully controlled study, adolescents consumed more food in the evening, especially those with higher body weight. This suggests that obesity may be influenced by internal biological processes, not just lifestyle choices.

The Link Between Obesity, Eating Patterns, and Circadian Rhythms

Obesity is a complex condition influenced by multiple factors. While research has established links between sleep, eating habits, and weight gain, the exact role of the circadian system — our internal biological clock — in regulating eating patterns remains unclear.

However, a new study from the Warren Alpert Medical School of Brown University and Mass General Brigham has identified a strong connection between circadian rhythms, weight, and eating behaviors in adolescents. This age group is particularly important because their eating patterns can have long-term health implications. The study found that adolescents classified as overweight or obese consumed more calories later in the day compared to those with a healthy weight.

Circadian Rhythms and Their Role in Eating Behavior

According to lead investigator Mary Carskadon, a professor of psychiatry and human behavior at Brown University, these findings highlight the significant role circadian rhythms play in influencing late-day calorie intake, particularly in individuals at higher risk for obesity.

“The critical nature of adolescent development to set the stage for a lifetime of health highlights the need to understand the roles played by sleep/wake and circadian timing processes for eating behavior,” said Carskadon, who also directs the Bradley Hospital Sleep Research Laboratory. “The knowledge gained here opens a door to potential interventions that can enhance teen health moving forward.”

The results were published today (February 17) in the Proceedings of the National Academy of Sciences.

Understanding the Body’s Internal Clock

The circadian system is composed of trillions of “clocks” present in virtually all organs, tissues, and cells, which prepares biology and behavior to adapt to the changing demands across the day/night cycle. The influence of the circadian system is known to differ between people due to a combination of genetic, behavioral, and environmental factors.

Previous studies have analyzed sleeping and eating behavior using measures of self-reported hunger and other variables, said study author David Barker, an associate professor of psychiatry and human behavior (research) at Brown. Some factors that set this study apart, Barker said, were the meticulous measuring of food before and after meals and the fact that environmental and behavioral influences were controlled for while the participants remained in the lab.

A Controlled Environment: No Clocks, No Light, Just Science

Fifty-one volunteers between the ages of 12 and 18 participated in the study, which was conducted at the Bradley Hospital Sleep Research Laboratory. Participants were divided into three groups based on body mass index. They were placed on 28-hour sleep and wake cycles — slightly longer than a typical 24-hour day — and stayed in a controlled dim-light setting while awake and in complete darkness during sleep.

Participants remained in the same space throughout the study, which lasted for 11 days and 10 nights. To control for outside influences on circadian rhythm, researchers removed all external time cues from the lab’s environment, including clocks and access to natural light.

Participants received six opportunities to eat at fixed times across the wake episode, with a standardized menu and could consume as much food during the meal as they wanted. Researchers tracked the food that was eaten as well as caloric intake. During the day, they were monitored by student research apprentices from Brown University and other institutions, who offered opportunities to join in a variety of activities including making crafts, watching movies (with screen lights dimmed), and playing social games.

Late-Night Calories and the Circadian Clock’s Influence

The results showed that changes in the circadian system throughout the day and night significantly influenced food consumption. In all three groups, food intake peaked in the late afternoon and early evening and was lowest in the morning, even after accounting for behavioral and environmental factors, demonstrating that the body’s biological clock directly impacts how much people eat at different times of the day.

Adolescents in the obesity and overweight groups consumed significantly more calories in the circadian evening compared to those in the healthy weight group. Researchers found no significant differences in total sleep time between or within the groups across sleep cycles.

According to study author Frank Scheer, director of the Medical Chronobiology Program at Brigham and Women’s Hospital, it was already well-known that the circadian system affects hunger and metabolism. What remained unclear, he said, was whether the circadian system — when isolated from influences of environmental and behavioral cycles, including light, sleep, and activity cycles — directly influences food consumption.

“This study is the first to demonstrate that food intake itself is regulated by our internal body clock,” Scheer said.

Shaping the Future: Circadian Rhythms and Weight Management

Future studies are needed to determine whether affecting circadian control of food intake contributes to weight changes, if weight changes impact the circadian control of food intake, or a combination of the two.

Carskadon said that the findings like those from the study could inform clinicians counseling adolescents on ways to manage their weight.

“For example, the influence of circadian timing could be influenced by excluding light late in the day and enhancing bright light in the morning, especially while exercising,” Carskadon said. “That might help pull the rhythms to a better place — and also create healthy habits.”

With future research, the team aims to gain a deeper understanding of the interactions between the circadian system, diet, and metabolism, as well as the mechanisms underlying these relationships, and the implications for developing timed dietary interventions to improve health.