Obesity and the biological clock: When times are out of joint

Three temporal cycles shape our lives. Our biological clock ensures that fundamental physiological processes oscillate with a period of approximately 24 hours. This internal timekeeper used the daily succession of light and dark to synchronize to the 24-hour day on our planet. 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 biological clock, 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 perturbed, often leading to overeating.

And it is not just sleep duration that is important here. The LMU team has 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.

Moreover, it appears that the incidence of social jetlag is itself increasing, perhaps as a consequence of a general reduction in sleep duration."The ongoing debate on the usefulness of daylight-saving time (DST) should take note of our findings," remarks 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.(suwe)