Social jet lag

If your weekends run on later nights and long lie-ins, you may know the Monday that follows: sleep is slow to come on Sunday night, the alarm goes at the usual time, and your body behaves as though it's still the middle of the night. It eases as the week goes on, and the weekend sets it off again.

This is what we term social jet lag, and the same biology sits underneath it as under the jet lag you get from flying.

The clock that sets your day

Deep in the brain, just above where the nerves from the eyes cross, sits a small cluster of cells called the suprachiasmatic nucleus (the body's master clock). It runs on a cycle of roughly a day and times when you feel alert, when melatonin (the hormone that signals night) starts to rise in the evening, and when sleep comes most easily.

Left to itself the clock drifts, so it's reset every day, mainly by light reaching the eyes. Light soon after waking moves it earlier; light late in the evening moves it later. This is why your waking time matters so much: when you get up largely decides when your eyes first see daylight, and therefore where the clock settles.

How the weekend moves the clock

During the week the alarm fixes your waking time. At the weekend, bedtime and waking may both move later, sometimes by two or three hours. Your first light of the day arrives later, your evening light runs later, and the clock follows both, drifting later over the two days off.

On Monday the alarm pulls you back to the weekday timetable, but the clock can't jump back in one go. It shifts a little each day under the influence of light, so for the first part of the week you're getting up while your body is still set to a later time zone. Flying works the same way: cross several time zones and your watch changes at once, while your body clock catches up over days. Travel-induced jet lag and social jet lag differ only in what moves the timetable, the aeroplane in one case and your own weekly routine in the other.

What the research shows

The term comes from Wittmann and colleagues (2006), who surveyed 501 volunteers about their sleep on work days and free days. Late types, people whose clocks naturally run later, showed the biggest gap, building up a sleep debt in the working week and making it up on days off. Late timing went with poorer wellbeing and more use of caffeine, nicotine and alcohol, most strongly in people up to 25, and the authors judged the gap between the timetables the more likely explanation than being a late type as such. As a questionnaire study, it shows these things travel together without showing what causes what.

Phillips and colleagues (2017) measured the clock directly. They followed 61 university students for 30 days with sleep diaries, then compared the most and least regular fifths. In the least regular, the evening rise in melatonin came about two and a half hours later. A model of how light acts on the clock suggested the difference came largely from their different patterns of light exposure.

There's also a longer-term signal. Huang and colleagues (2020) gave 1,992 mostly older adults a wrist tracker for a week and followed them for a median of almost five years. Those whose time of falling asleep varied by more than an hour and a half from night to night had about twice the rate of heart and circulation problems of those who varied by half an hour or less. The groups in between weren't clearly different, and because the study was observational we can't say a steadier routine would have prevented those problems.

How light sets the clock, and that irregular timing moves it later, we can state flatly. How much a steadier routine changes your health, or how you feel day to day, is far less settled, and it will vary from person to person.

Steadying the week

Your sleep diary shows how far your waking time moves between work days and days off. If the gap is large, the plan will usually ask you to bring your days-off waking time closer to your work-day one, in small steps, and to get daylight soon after you get up so the clock gets a clear morning signal.

If you're short of sleep in the week, the plan will also look at the length of your work nights, because a long lie-in is often a sign that weekday nights are too short. Your diary and your morning ratings over the following weeks will show whether a steadier start makes Mondays easier for you.