Does sleeping in on weekends make up for lost sleep?

Partly. It pays back some lost sleep, but it also shifts your body clock. Sleeping in about three hours on Saturday and Sunday can move your body clock around 40 minutes later by Sunday evening. That's from a 2008 study by Taylor, Wright and Lack. They measured when people's melatonin rose on Sunday evening after sleeping in, and compared it with a weekend of normal wake times ¹. So if you normally go to bed at 11 p.m., 11 p.m. on Sunday feels more like 10:20 p.m. After the sleep-ins people took a little longer to fall asleep on Sunday night and reported more sleepiness and fatigue on Monday and Tuesday ¹.

What is social jetlag?

Marc Wittmann, Till Roenneberg and colleagues coined the term social jetlag in 2006 for the gap between when you sleep on workdays and when you sleep on free days ². It is measured by comparing the midpoint of sleep on free days with the midpoint on workdays. If you sleep from midnight to 6:30 a.m. on weekdays and 1 a.m. to 10 a.m. on weekends, the midpoints are 3:15 a.m. and 5:30 a.m., a gap of a little over two hours, roughly like flying two time zones west for the weekend and back on Monday.

In that first study of about 500 people, later chronotypes had the biggest gaps, because the times they naturally slept were furthest from a normal working day ². Later work by Roenneberg's group drew on tens of thousands of questionnaire responses and found social jetlag was common, and that among people who were already overweight, more social jetlag went with a higher body mass index, even after accounting for how long they slept ³.

Can you catch up on sleep on the weekend?

If you've been short on sleep all week, sleeping longer on the weekend helps some, but studies disagree on how much. In a large Swedish cohort followed for 13 years, people under 65 who slept five hours or less on weekdays but longer on weekends had no higher mortality than people who slept six to seven hours every night. People who slept short on both had higher mortality ⁴. The authors suggested longer weekend sleep may make up for short weekday sleep, though an observational study like this can't prove it.

Lab studies found less benefit. In Depner and colleagues' 2019 trial, young adults slept five hours a night for a working week, then could sleep as much as they wanted for a weekend, then went back to short nights. They slept only about an extra hour in total across the weekend. Once the short nights resumed, their body clock had shifted later, they ate more after dinner, and their insulin sensitivity dropped, by about as much as in a group that never got the weekend ⁵. Belenky and colleagues found earlier that people restricted to short sleep for a week had not fully recovered their reaction time after three nights of normal sleep ⁶.

Does morning light on the weekend help?

In older teenagers, Crowley and Carskadon found that a later weekend schedule delayed the clock by about 46 minutes, and adding a session of blue light after the late wake-up did not prevent it ⁷. A later lab study in teens found that two and a half hours of bright morning light on the weekend moved their clocks earlier by about an hour ⁸. That was in teenagers, and it is far more light than a normal Saturday morning gives. For how light timing works on an ordinary morning, see the morning sunlight guide.

How to catch up on sleep without shifting your body clock

If you wake within an hour or so of your weekday time, your clock usually moves less. If you are very short on sleep, go to bed earlier on Friday and Saturday instead, so you get more sleep but still wake up and get light at your usual time. Get outside soon after you wake on both days. A short nap in the early afternoon can also help with sleepiness ⁹ without moving your wake time.

Late Friday and Saturday nights under bright lights and screens also push your clock later, so dim the lights late on those nights. The evening light guide explains what to change at home. If you can't sleep at your usual time on Sunday, keep the lights low, get up at your normal time on Monday anyway, and expect to feel a bit tired for the first day or two.

If your weekday schedule leaves you so short that you need very long weekend sleep every week, you need more sleep on weekdays, usually by moving bedtime earlier. Fixing your sleep schedule with light timing covers how to move it earlier step by step.