Sleep glossary
Social jetlag
The mismatch between your body clock and your calendar, how to work out your own number in one subtraction, and why Monday keeps hurting.
In short
Social jetlag is the gap between the sleep your body clock wants and the sleep your schedule allows, measured as the shift in your mid sleep time between work days and free days. Two hours of it is like flying two time zones every week.
Social jetlag is also called social jet lag or weekend jetlag.
The number is easy to work out. Take the midpoint of your sleep on a work night: bed at 23:30, up at 06:30, midpoint 03:00. Then take it on a free night: bed at 01:30, up at 10:00, midpoint 05:45. The difference, two hours and forty five minutes, is your social jetlag. No app required, though Tock tracks the related consistency numbers among its 21 sleep metrics if you would rather not do the arithmetic every week.
What makes it worth naming is that your body treats the shift the way it treats travel. Your circadian rhythm spends Saturday and Sunday drifting later, because you are getting light later and sleeping later, and then Monday demands it snap back. Nothing about the clock snaps. It moves in small steps, which is why Monday morning feels like a bad landing and Wednesday is when you finally feel normal.
Who carries the most of it
Late chronotypes on early schedules, by a distance. If your clock wants 01:00 and your job wants 06:30, you lose sleep every work day, and the weekend lie in is not indulgence, it is repayment of real sleep debt. The trap is that the repayment and the problem are the same act: sleeping in restores some of the lost hours while pushing the clock further from the schedule you have to meet on Monday.
The way out is unglamorous. Hold your wake time inside about an hour of your weekday time, even on Saturday. Get outside soon after waking, since morning light advances the clock and is the strongest tool you have. Then let bedtime follow, rather than trying to force it. If you want the sequence laid out properly, how to fix your sleep schedule covers what to change first and how long to give it. The measure that captures this across a whole week, rather than two nights, is the sleep regularity index.