"We saw people leading different lives on weekends and workdays," he says on a call with mbg. On weekends, many people slept at times that differed greatly from those during the workweek, as if “they were flying from Europe to the States on Friday evening and flying back to Europe from the States on Monday morning. That's where the term 'social jet lag' came from."
You might be thinking, why does it matter if you go to bed later (and wake up later) on weekends—as long as you're still getting the same amount of sleep? Roenneberg has found that even if people clock their eight hours of on the weekends, social jet lag still has potential health consequences because of the way it puts them out of sync with their circadian rhythms.
These biological clocks dictate the time window when each one of us naturally wants to spend asleep and awake. Someone with an early rhythm—or chronotype—is known as an early bird or lark, while someone with a late chronotype is more of a night owl. Where we fall on this spectrum is the result of our genetics, age (most people have the latest chronotype during adolescence, and it gradually gets earlier from there), and light conditions, Roenneberg explains.
When we travel across time zones, those lighting conditions change dramatically, and our clocks need a few days to catch up. "But unlike travel jet lag, social jet lag doesn't stop," Roenneberg says. "It's chronic."
You can think of social jet lag as the gap between when our bodies naturally want to sleep and when our social schedules allow us to sleep. If you're someone who needs to set an alarm every morning of the workweek and are exhausted come Fridays, you already know what this gap feels like. You're constantly awake during times when your body wants you to be asleep—and have the caffeine dependency to prove it. Switching your sleep schedule on the weekends will only confuse your circadian rhythm more, making it harder to return to your alarm clock on Monday morning.
Roenneberg explains that the later your chronotype, the more likely you are to suffer from social jet lag since many workplaces and schools start early in the morning, and we are generally told that it's important to wake up early to get a head start on the day. "The later you are, the more likely it is that the social times are forcing you to live against your body clock," he says.
Since it was first coined in 2006, social jet lag has appeared in over 200 academic papers. "It's a concept that has conquered the world unexpectedly," Roenneberg laughs. Marc Wittmann, Ph.D., a time perception researcher who has worked alongside Roenneberg, has a guess as to why it's become so popular: "It's because we all deal with it."