Free tool · Sleep
Social jet lag calculator
Short answer
Social jet lag is the gap between the middle of your sleep on free days and on work days. Asleep 11:00 PM to 6:30 AM on work days and 12:30 AM to 9:30 AM on free days puts mid-sleep at 2:45 AM and 5:00 AM: a social jet lag of 2 h 15 min. A review puts about 70% of students and workers at an hour or more.
Last reviewed 28 October 20262 published sourcesNot medical advice
Reps keeps track of this for you on iPhone, iPad and Apple Watch.
Get Reps freeSocial jet lag calculator
Use the time you actually fall asleep (not lights out) and the time you wake up, on a usual work or school day and on a usual free day with no alarm.
Choose when you fall asleep and wake up on work days and on free days.
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| Free-day sleep | Free-day mid-sleep | Social jet lag |
|---|---|---|
| 11:00 PM–7:00 AM | 3:00 AM | None |
| 11:30 PM–8:00 AM | 3:45 AM | 45 min |
| 12:00 AM–8:30 AM | 4:15 AM | 1 h 15 min |
| 12:30 AM–9:30 AM | 5:00 AM | 2 h |
| 1:30 AM–10:30 AM | 6:00 AM | 3 h |
| 2:00 AM–11:30 AM | 6:45 AM | 3 h 45 min |
How social jet lag is measured
Wittmann and colleagues described the discrepancy between work and free days, between social and biological time, as “social jetlag” (Wittmann et al., 2006). It is calculated as the absolute difference between mid-sleep on free days (MSF) and mid-sleep on work days (MSW) (Caliandro et al., 2021).
Mid-sleep is the halfway point between falling asleep and waking. Use the time you actually fall asleep, not when you turn the lights out, and free days when you wake without an alarm.
In a 2021 review, about 70% of students and workers had at least one hour of social jet lag, and almost half of them two hours or more (Caliandro et al., 2021).
Plan steadier bedtimes with the sleep calculator, or find out whether you lean morning or evening with the chronotype quiz.
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Common questions
What is social jet lag?
Wittmann and colleagues coined the term in 2006 for the mismatch between social time (alarms, work and school) and biological time. It is measured as the absolute difference between your mid-sleep on free days and on work days.
How do I calculate mid-sleep?
Mid-sleep is the clock time halfway between falling asleep and waking. Asleep at 11:00 PM and awake at 7:00 AM is 8 hours of sleep, so mid-sleep is 3:00 AM.
How common is social jet lag?
A 2021 review reported that about 70% of students and workers have at least one hour of social jet lag, and almost half of them two hours or more.
Sources
Figures on this page were checked against these publications on 28 October 2026.
- Wittmann M, Dinich J, Merrow M, Roenneberg T. Social jetlag: misalignment of biological and social time. Chronobiology International. 2006;23(1–2):497–509. doi.org/10.1080/07420520500545979
- Caliandro R, Streng AA, van Kerkhof LWM, van der Horst GTJ, Chaves I. Social jetlag and related risks for human health: a timely review. Nutrients. 2021;13(12):4543. doi.org/10.3390/nu13124543
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