Obesity, Social Jetlag and Our Natural Rhythms: When Our Internal Clock is Out of Synch

Urgent appointments, tight work timetables and hectic social schedules structure modern life, and they very often clash with our innate biological rhythms. The discrepancy results in so-called “social jetlag,” which can damage our health. Among other effects, researchers now believe it can contribute to the development of obesity, according to a new study from the Ludwig-Maximilians Universitat Munchen (LMU) in Germany.

The researchers explain that three “temporal cycles” shape our lives. These circadian rhythms ensure that fundamental physiological processes occur within a period of roughly 24 hours. This internal timekeeper uses the daily cycles of light and dark to synchronize to the 24-hour day. Our social clock, on the other hand, often takes little or no heed of our natural needs and biological rhythms. The beat of the social clock is determined by the demands of our work schedules and other extraneous timetables, and its timekeeper is the trill of the alarm clock.

“Our surveys suggest that in Western societies two thirds of the population are burdened with a significant discrepancy between their internal time and the demands imposed by school and work schedules and leisure stress,” says LMU chronobiologist Professor Till Roenneberg, who coined the term “social jetlag” to describe the phenomenon. If the rhythms dictated by our lifestyles are persistently out of phase with our natural rhythms, the risk of illness, such as high blood pressure and even cancer, rises.

Tired – around the clock

A team of researchers led by Roenneberg has now shown that social jetlag also contributes to another growing health problem, particularly in countries with a Western lifestyle – obesity. Individuals who are overweight are at increased risk for serious metabolic diseases, such as diabetes. Many factors, in addition to excessive consumption of energy-rich foods, play a role in the development of obesity, and one of them is a lack of sleep. In persons who get too little sleep, the perception of hunger is increased, often leading to overeating.

And it is not just sleep duration that is important, say the LMU researchers. The research team  also found that social jetlag shows a significant association with increased body-mass index (BMI). The BMI, which is based on a quantitative relationship between weight and height, is used as a measure of body fat, and varies depending on age and sex. Individuals with BMIs above the normal range are regarded as being overweight or obese. The results of the new study strongly indicate that a lifestyle that conflicts with our internal physiological rhythms can promote the development of obesity.

The researchers suggest that the incidence of social jetlag is itself on the rise, perhaps due to an overall reduction in the time people spend sleeping.”The ongoing debate on the usefulness of daylight-saving time (DST) should take note of our findings,” says Roenneberg. “Just like conventional school and work schedules, DST disrupts our biological clock and subjects us to more social jetlag with all its consequences.”

The data used in the new epidemiological study are based on responses to a standardized set of questions, known as the Munich ChronoType Questionnaire. This is freely available on the internet, and more than 130,000 people have already filled it out, providing detailed information on their sleeping patterns and other aspects of their normal lifestyles on working days and on weekends. You can fill out the questionnaire here to assess your own risk for social jetlag.

Publication: Social jetlag and obesity. Till Roenneberg, Karla V. Allebrandt, Martha Merrow, and Céline Vetter Current Biology online, 10.5.2012