For some of us, a weekly case of the Mondays isn’t just because of traffic, work pileups or our soulless office space. It’s because we had to get up early, and sleeping in on the weekend was so incredibly glorious. Besides, because we slept in on Sunday, we didn’t get to the gym until the afternoon, we cooked a late dinner for a friend and then we couldn’t fall asleep at all and so stayed up playing around on the Internet. OK, maybe that’s just me. But you get the general idea.
Our obligations — work, family and friends — often don’t line up with when our bodies want to sleep. Scientists call this phenomenon social jetlag. And it may make for more than just miserable Mondays. Social jetlag may also be associated with wider waistlines. As we learn more about how our body clocks work, it might help to think about how our own schedules can shift.
Some of us love late nights and can’t help glaring at those who hop out of bed for a 5 a.m. workout (again, maybe that’s just me). But in fact our chronotypes aren’t a result of willpower. Instead they fall in a natural curve. About two-thirds of people are neutral, but a few fall at each end of the spectrum, rising extra early, or staying up until the wee hours.
But even those in the middle are still getting up a little bit too early and staying up a little bit too late. We try to make up for it on days off, sleeping in or falling asleep early for a few extra hours of rest. But the result of that shift in sleep schedule? Jetlag. “It’s the equivalent of taking a flight one direction every Friday and going back every Sunday,” says Michael Parsons, a behavioral geneticist at the Medical Research Council Harwell in England.
Social jetlag isn’t just about being groggy. Using a group of more than 1,000 New Zealanders from the town of Dunedin, followed throughout their lives and questioned regularly about their health, Parsons and other scientists showed that as little as a two-hour difference between weekday and weekend sleep schedules was associated with a higher body mass index, compared with people with no social jetlag. “It was an additional two [kilograms] of fat mass at age 38,” Parsons says. The authors published the work January 20 in the International Journal of Obesity.
The association was fairly modest, but it replicates a previous finding from Till Roenneberg at the Ludwig-Maximillian University in Munich. With data from a massive online questionnaire of more than 65,000 people, Roenneberg showed for the first time that social jetlag was associated with obesity. The work was published in 2012 in Current Biology.
Forced early rising and small amounts of sleep deprivation can have effects on more than just your coffee cravings. Parsons calls social jetlag “a subtle but chronic stressor.” People (and mice) getting less sleep eat more. Not only that, they eat more at night. At night, leptin levels are high, inhibiting hunger. But when circadian clocks are misaligned, leptin gets lower, and night time feeding increases. Sensitivity to insulin is lower when we are active and higher at rest. People who sleep less produce more of it, a sign of insulin resistance and a precursor to diabetes.
Of course, Parsons notes, our sleep hours aren’t the only contributor to obesity. There are other environmental aspects, genetics and behavior to consider when it comes to how we eat and what our body does with the food once it arrives.
Not only that, there’s not even just one clock to blame. “We have trillions of clocks in our bodies,” says Frank Scheer, a circadian physiologist at Harvard Medical School. “Different clocks are differentially sensitive to different cues.” The “central” clock, located the in suprachiasmatic nucleus of the brain, is most sensitive to light. But metabolic signals, he says, are more sensitive to food timing cues, reacting predictably at times when the body is used to eating. When light and food intake get out of sync, the system begins to fall apart, Scheer explains: “Your master clock is on Boston time, but your liver might be in Tokyo.”
But when it comes to social jetlag, it may help to retrain at least one part of your system to favor earlier hours. If you want to advance your central clock to get up a little earlier, Scheer says, get as much exposure to bright sunlight in the early morning as you can. Similarly, you should decrease exposure to light at night, especially light from our favorite electronic screens. Studies are ongoing in several labs to find out how much this experimental shifting, as well as shifting when people eat and how much, might help our bodies deal with our modern lifestyle.
And of course, consistency is key. “During the work or school week people are forced to be awake very early. On the weekends they are less restrained” by their schedules Scheer says, and more inclined to sleep in, setting the social jetlag in motion for Monday morning. So early morning light exposure will help, but so will not sleeping in on the weekends. Scheer has a pretty consistent sleep-wake schedule, but necessarily by choice. “I don’t have a lot of social jetlag,” he admits. “I have two young boys, so no sleeping in for me.”