Sleeping differently on weekends: social jet lag and catch-up sleep

What social jet lag is and how it's calculated

The term was proposed in 2006 by a team at the University of Munich to describe the clash between the schedule imposed by work or school and the one each person would follow without obligations.¹ It's calculated with the midpoint of sleep, the time halfway between falling asleep and waking up: social jet lag is the difference between that midpoint on days off and on workdays.²

It tends to be larger in people with a late chronotype, those who without an alarm would tend to fall asleep and wake up later: during the week the alarm cuts off the end of their night, and on days off they sleep longer and later. It peaks in adolescence and declines with age, although it remains present throughout working life.²

A nuance pointed out by the very authors of the concept: the name suggests that the internal clock stays put while the social schedule moves, but the markers of the body clock, such as melatonin, shift fairly quickly when schedules change; according to the studies they cite, by about 45 minutes after just two days. That's why they suggest also seeing it, in part, as a small round trip every week: on days off your pattern of light and darkness moves, and when you come back the clock has to readjust, although they point out that the solar day doesn't change and that it's more like shift work than like travel.²

What a large sample in Spain reported

Spain uses Central European Time, although its geographic position would correspond to Western European Time.³ In Galicia, for example, the official clock is an hour and a half out of step with the sun.²

A team at the University of Murcia collected, between 2016 and 2023, the schedules of 9,947 people living in Spain, with an average age of 35, who voluntarily answered an online questionnaire. On workdays they got up about two hours earlier than on days off and went to bed an hour earlier.³

The difference between the two midpoints, a version of social jet lag calculated with the time of going to bed instead of the time of falling asleep, averaged an hour and a half. And it was larger the younger the group: a little over two hours between ages 13 and 17 and almost two (1.8) between 18 and 30, compared with a little over an hour between 51 and 64.³

Can you catch up on sleep over the weekend?

It depends on what's meant by catching up. There are two separate questions: whether the weekend gives back the hours that were missing, and whether it undoes what those missing hours do to the body.

A laboratory experiment at the University of Colorado tested it with 36 healthy young adults who spent about two weeks staying in the lab. One group had 9 hours in bed every night; another, 5 hours every night; and a third, 5 hours from Monday to Thursday, a Friday on which they went to bed two hours later but got up whenever they wanted, a weekend on a free schedule and then two more short nights.⁴ On Friday and Saturday nights, this last group slept a total of about 3 hours more than at the start of the study; but on Sunday, knowing that “Monday” meant an early start, they slept less. Overall they recovered a little over an hour, against the more than twelve lost during the week.⁴

On their days off they got up between three and a half and almost four hours later than usual, and their body clock, measured with melatonin, ended up more delayed than at the start: the end of the biological night came later once they were back in the workweek.⁴ When they went back to short sleep, their insulin sensitivity, a measure of how the body handles sugar, also got worse, as had already happened to the group with no weekend, and they again ate more after dinner.⁴

Outside the laboratory, the picture is more nuanced. An expert consensus of the National Sleep Foundation concluded in 2023 that regular schedules are important for health but that, when people sleep too little during the week, catching up on sleep on days off can be beneficial.⁵ And in a Swedish cohort of 38,015 adults followed for 13 years, among those under 65 who slept 5 hours or less on weekdays but longer on days off, mortality was no different from that of the reference group (those who slept 6 or 7 hours every day); it was higher among those who slept little every day.⁶ It's an association, with sleep asked about only once at the start and few people in that group, and it doesn't prove that sleeping more on weekends makes up for anything.

Our overall reading, which isn't the conclusion of any particular study: sleeping more on days off eases part of the sleep shortfall, but it doesn't give back all the hours, and if it's done by getting up much later, it shifts the clock right before Monday. It's a useful fallback, not a substitute for getting enough sleep during the week.

What irregular schedules are linked to

Switching schedules between weekdays and weekends is one specific form of irregularity. Research on it is almost all observational: it compares people with more or less regular schedules and looks at what else happens to them.

• Weight and body measurements. A meta-analysis of 43 studies, with 231,648 people, found that greater social jet lag is associated with a somewhat higher body mass index and waist size. The associations were small, the studies very different from one another and almost all cross-sectional, and the authors rate the certainty of the evidence as very low for almost all measures.⁷
• Regularity and mortality. Among 60,977 UK Biobank participants, with an average age of 63 and one week of recording with an accelerometer, those with more regular schedules had lower mortality in the following years, and regularity predicted it better than sleep duration did.⁸ The authors acknowledge that regularity may be a cause or just a marker of other things.
• Students. Among 61 students at a US university followed for 30 days, those with the most irregular schedules slept the same total amount as the most regular ones, because they made up for it by sleeping during the day. But, comparing the 12 most irregular with the 12 most regular, their body clock ran about two and a half hours later and their light exposure had less contrast between day and night; regularity was also linked to better grades.⁹

What margin seems reasonable

No study has established how much difference between weekdays and weekends is acceptable. What follows are our own inferences from the sources, not recommendations tested in trials.

• Look first at your weekday sleep shortfall. If every Saturday you need to sleep much longer, that usually points to a debt built up during the week.² Getting more sleep during the week tackles the cause; the weekend only makes up for part of it.⁴
• If you sleep longer, don't do it only in the morning. In the laboratory, the extra sleep on days off came mostly from getting up much later, and the clock was delayed.⁴ Going to bed a little earlier or taking a nap moves the time you get up less; when and for how long a nap is worth it we cover in the article on naps.
• A window for the time you get up. In the UK Biobank, the 20% with the most regular schedules fell asleep and woke up within a one-hour window on most days, and the least regular 20%, within about three hours; the authors propose that one-hour window as a feasible goal.⁸ Take it as guidance, not as a rule: it hasn't been tested in a trial.
• Daylight when you get up, on weekends too. Social jet lag largely measures how much your pattern of light and darkness moves,² and an expert consensus recommends plenty of light during the day, preferably natural.¹⁰ Getting out into the morning light on days off is a simple way to keep the clock from moving as much; it's an inference, not a measured result.

To find out whether a change works for you, keep it up for a few weeks and write down your times, in a diary or with a watch. If you use a device, it's worth knowing what it can estimate about your sleep and what it can't; and to set up the test without fooling yourself, it helps to turn the data point into a question and a test.

If daytime sleepiness or trouble adjusting to your schedules lasts for months or affects your daily life, see a health professional; in the general article on sleep we explain when it's worth doing so.