What is social jet lag?

Where the idea comes from

The term was coined by Marc Wittmann, Till Roenneberg and colleagues in a 2006 paper. It grew out of the Munich ChronoType Questionnaire, which Roenneberg's group had built a few years earlier, and which did one thing other sleep questionnaires did not: it asked about workdays and free days separately.

That separation turned out to matter. In the first 500 questionnaires, reported in 2003, sleep times differed considerably between the two kinds of day for everyone except the very earliest types. Later types, in particular, built up a sleep debt during the working week and then lengthened their sleep by several hours on free days to make up for it. The same pilot study found that people who spent more time outdoors in daylight tended to sleep earlier.

In a 2019 review of the questionnaire's history, the group describes reading sleep logs that looked as if their subjects were flying west on Friday evening and returning on Monday morning, without ever travelling. That picture gave the idea its name.

How it is measured

The questionnaire takes the midpoint of sleep, halfway between falling asleep and waking, because mid-sleep is one of the more accurate behavioural markers of where someone's internal clock sits. Mid-sleep on free days is used as the measure of chronotype, the term for where you fall between early and late. Mid-sleep on workdays shows where your schedule has placed you instead.

Social jet lag is the distance between the two. Take the example above. Sleeping from 00:30 to 07:00 puts the workday midpoint at 03:45. Sleeping from 02:30 to 10:30 puts the free-day midpoint at 06:30. The difference is two hours and forty-five minutes, and it repeats every week.

The original definition used the absolute difference. The 2019 review notes that the signed difference is also informative, because a small number of people sleep later on workdays than on free days, which gives a negative value. Most people's numbers run the other way.

Why late types carry most of it

The weekend sleep schedule is not really the cause. It is where the underlying mismatch becomes visible. For an early type, the alarm clock and the body's own waking time are close together, so workdays and free days look much alike. For a late type, the night gets squeezed from both ends during the week: the body's clock does not open the window for sleep until late, and the alarm closes it early. On free days the sleep moves back toward where the clock would put it anyway.

Wittmann's 2006 study of 501 volunteers found that late chronotypes showed the largest differences between work and free days, and that these differences built up a considerable sleep debt on workdays, which was then repaid at the weekend. The study also reported associations between social jet lag, wellbeing and the use of stimulants such as caffeine and nicotine, strongest in teenagers and young adults up to 25. The authors argue that these links most probably follow from social jet lag rather than from chronotype alone, but the data are questionnaire correlations, which describe what tends to travel together rather than proving what causes what.

The review is candid about what remains unsettled. Its authors ask whether social jet lag measures the clock itself or something about how the clock moves under a weekly pattern of light, and say plainly that the question is still open.

What the number is good for

The useful thing about social jet lag is that it turns a vague feeling into a single figure. Many people sense that Monday mornings arrive too early, and that the first days of a week are harder than the last. The measure gives that feeling a size, and shows that it belongs less to the person than to the fit between their clock and their calendar.

It also explains why advice about sleeping in at the weekend tends to be argued in both directions. A long Saturday morning repays part of the debt, and it is also the free-day midpoint that the measure counts, so the gap is there again when Monday's alarm goes.

None of this is medical advice. If tiredness during the week is affecting your days, a doctor is the right person to talk to. If you would like to know where your own mid-sleep sits, the free chronotype test takes about two minutes and asks about workdays and free days separately, for the reasons above. And if you are curious how differently people once divided the night, the night of two sleeps is a slow, true story about exactly that.