The causes of obesity are complex and influenced by many factors. While research has shown links between sleep, eating habits and weight gain, scientists are still unsure about the role of the circadian system – the biological clock – in shaping eating habits. However, a new study from researchers at Brown University’s Warren Alpert Medical School and Mass General Brigham shows a clear link between circadian rhythms, weight and eating habits in adolescents, a vulnerable age group whose eating habits affect their lifelong health. The study found that teens who were classified as “overweight” or “obese” by weight status consumed more calories later in the day compared to participants of a healthy weight.

The findings, published in Proceedings of the National Academy of Sciences, suggest that the circadian rhythm plays an important role in explaining later calorie intake in individuals at risk for obesity, according to lead researcher Mary Carskadon, professor of psychiatry and human behavior at Brown University’s medical school. “The critical importance of the adolescent developmental period for setting the stage for a lifetime of health underscores the need to understand the role of sleep/wake and circadian timing processes on eating behavior,” said Carskadon, who also directs the Sleep Research Laboratory at Bradley Hospital. Knowledge gained here opens the door to potential interventions that may improve the health of teens in the future.

Biological Clock and Eating Behavior

The circadian system consists of trillions of “clocks” present in virtually all organs, tissues, and cells, priming biology and behavior to adapt to the changing demands in the day-night cycle. The circadian system is known to affect people differently, due to a combination of genetic, behavioral and environmental factors. Previous studies have analyzed sleep and eating behavior using self-reported measures of hunger and other variables, according to study author David Barker, associate professor of psychiatry and human behavior (research) at Brown University.

Some factors that set this study apart, Barker said, were the careful measurement of food intake before and after meals and the fact that environmental and behavioral influences were controlled while the participants stayed in the lab. Fifty-one volunteers between the ages of 12 and 18 took part in the study, which was conducted at the sleep research laboratory at Bradley Hospital. The participants were divided into three groups based on their body mass index. They were placed on 28-hour sleep and wake cycles – slightly longer than a typical 24-hour day – and while awake, they remained in a controlled environment with dimmed lights and in total darkness while sleeping. Participants remained in the same room throughout the study, which lasted 11 days and 10 nights. To control for external influences on circadian rhythm, the researchers removed all external cues from the lab environment, including clocks and access to natural light.

During waking hours, the participants were given six opportunities to eat at set times, with a standardized menu that allowed them to eat as much as they wanted during the meal. Researchers tracked the food eaten as well as calorie intake. During the day, they were monitored by research students from Brown University and other institutions who provided opportunities to participate in various activities, including crafts, watching movies (in dim lighting) and playing board games.

The results showed that changes in the circadian system during the day and night significantly influenced food consumption. In all three groups, food intake was highest in the late afternoon and early evening and lowest in the morning, even when behavioral and environmental factors were taken into account. This shows that the body’s biological clock has a direct influence on how much people eat at different times of the day. Adolescents in the obesity and overweight groups consumed significantly more calories in the evening than those in the healthy weight group. Researchers found no significant differences in total sleep time between or within groups across sleep cycles.

Interactions Between the Circadian System, Nutrition and Metabolism

According to study author Frank Scheer, director of the Medical Chronobiology Program at Brigham and Women’s Hospital, it was already known that the circadian system affects hunger and metabolism. However, it was unclear whether the circadian system directly influences food consumption when isolated from the influences of environmental and behavioral cycles, including light, sleep and activity cycles. “This study is the first to show that food intake itself is regulated by our internal body clock,” Scheer said.

Further studies are needed to determine whether affecting circadian control of feeding contributes to weight change, whether weight change affects circadian control of feeding, or whether there is a combination of the two. Carskadon said the results of this study could serve as a source of information for doctors advising adolescents on weight control.

For example, the influence of circadian timing could be influenced by cutting out late-day light and increasing bright light in the morning, especially during exercise. This could help to shift rhythms to a better place – and also create healthy habits. With future research, the team hopes to gain a deeper understanding of the interactions between the circadian system, nutrition and metabolism, as well as the underlying mechanisms of these relationships and the implications for developing timed nutritional interventions to improve health.