Social jet lag is the gap between when your body wants to sleep and when your schedule makes you sleep. If you go to bed at midnight and wake at 7 on work days, but sleep from 2 to 10 at the weekend, the middle of your sleep moves by two and a half hours every Friday and back every Monday. To your body clock, that's close to flying from London to Moscow and back each week.
Where the idea comes from
The term was coined by the chronobiologist Till Roenneberg and colleagues, who described it in a 2006 paper with Marc Wittmann and others. Roenneberg's team collected sleep data from hundreds of thousands of people with the Munich ChronoType Questionnaire, and found that most people sleep at different times on work days and free days, and that a large share have a gap of an hour or more.
How to measure yours
- Find the midpoint of your sleep on work days. Asleep at midnight, up at 7: the midpoint is 3:30 AM.
- Find it on free days, with no alarm. Asleep at 2, up at 10: the midpoint is 6 AM.
- The difference is your social jet lag: here, two and a half hours.
Under an hour is common and mild. Two hours or more is a lot, and usually means you're short on sleep during the week and catching up at the weekend.
Why it matters
A weekly shift of body-clock time isn't harmless. Roenneberg's group found in a 2012 study that, among people who weren't already of normal weight, more social jet lag went with a higher body mass index, and other research has linked it with worse mood, more smoking and caffeine use, and poorer school performance. These are associations rather than proof of cause, but the pattern is consistent. And on Monday morning, the effect is easy to feel.
Who gets it most
Night owls, mostly. Their body clocks run late, so early alarms on work days fight their biology, and free days let them drift back. Teenagers and young adults, whose clocks naturally run later, are hit hardest by early school and work starts. The chronotype quiz gives you a sense of where you fall.
How to reduce it
- Keep weekend wake-up within an hour of weekday wake-up. The single most effective step. Nap after lunch if you need the sleep.
- Get morning light, especially on weekends. It pulls your clock earlier and anchors it.
- Dim the evenings. Bright screens late at night push the clock later.
- Get enough sleep on weeknights, so there's less to catch up on. The sleep calculator finds bedtimes that fit your alarm.
- Shift your schedule if you can. A later start, or keeping early meetings off your calendar, works with your clock rather than against it.
Jet lag, with a real flight
Real jet lag works the same way, just in one jump. If you're flying soon, the jet lag calculator shows how long your body needs. And if you'd rather fly at your desk, Flight turns the trip into a focus session, no time zones crossed.