One overlooked culprit in the world’s obesity epidemic may be the alarm clock, according to Till Roenneberg, a professor at the University of Munich’s Institute of Medical Psychology.
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He studies “social jet lag,” a term he coined, perhaps not surprisingly, on an airplane. But unlike the jet lag you get from shifting time zones, social jet lag is the chronic clash between what our bodies need (more sleep) and what our lives demand (being on time). And his research suggests that it’s playing havoc with our biological clocks.
In a study, published in May, Roenneberg and colleagues analyzed the sleep habits of more than 65,000 adults. Two-thirds of them suffered from social jet lag, experiencing at least a one-hour disparity between how long they slept on workdays and weekends.
The researchers also found that, over the past decade, people have been going to bed later but still getting up at the same time, losing about 40 minutes of sleep on workdays. They are also spending less time outside, which could account for why their circadian rhythms have become so late.
Previous studies have linked sleep deprivation with excessive weight, but Roenneberg’s team concludes that it isn’t just how much sleep people get that matters—it’s how much they mess with their internal clocks. For every hour of social jet lag accrued, the risk of being overweight or obese rises by about 33 percent. Obesity results from a host of influences, but Roenneberg says “one contributing factor is not living according to your biological temporal needs.” No one knows the precise mechanism, but other studies suggest that lack of sleep causes higher secretions of ghrelin, the appetite hormone, and a reduction of leptin, the satiety hormone.
Our daily lives are controlled by two naturally occurring phenomena: our internal circadian clock and the rotation of the earth. The hub of the body clock resides in a bundle of nerves called the suprachiasmatic nucleus, in the brain’s hypothalamus. This central clock acts as a pacemaker, synchronizing other cellular clocks that scientists believe exist throughout the body. This circadian clock system controls a variety of functions, including body temperature, hormone secretion and blood pressure. It also regulates the daily activities of organs.
The circadian clock must be rewound every day to keep it operating on a cycle of roughly 24 hours. It is reset by sunlight and darkness, the signals traveling to the brain through the optic nerve. Into that elaborate finely tuned natural system bursts the alarm clock.
Sleep is often viewed as an indulgence. But Roenneberg warns that people who sleep for fewer hours are not as efficient at their jobs, which creates a vicious cycle of working more and sleeping less. “Sleep has not been put out there by evolution as a time when we’re lazy,” he says. “ It’s a time when we’re preparing to be extremely active.”
Roenneberg doesn’t set an alarm clock unless he has to catch a plane, and he feels great. “I go through 16 hours without feeling a yawn,” he says.