How this calculator works
For each schedule, wake time is interpreted as the next occurrence after sleep onset, within 24 hours. Half of that interval is added to onset to find midsleep. The absolute workday/free-day midpoint difference is the uncorrected social jetlag measure used in sleep research. To handle times on either side of midnight, this tool uses the shortest difference around the 24-hour clock; an exact 12-hour difference is shown as -12 hours. This convention needs caution for rotating shifts. Use typical main sleep episodes, not naps, and enter sleep onset rather than time going to bed. The intervals include any awakenings between onset and final waking. This does not measure melatonin timing, diagnose a circadian disorder or assign a chronotype. The result is an estimate for information only and is not medical advice.
Formula
Sleep interval = (wake minutes - onset minutes) modulo 1440. Midpoint = (onset + interval/2) modulo 1440. Signed shift is the shortest free-day midpoint minus workday midpoint, from -12 to below +12 hours. Social jetlag = absolute signed shift.
Worked example
Workday sleep from 23:00 to 07:00 has midpoint 03:00. Free-day sleep from 01:00 to 09:00 has midpoint 05:00. Both intervals are eight hours, but free-day midsleep is two hours later, giving two hours of uncorrected social jetlag.
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Frequently asked questions
Is social jetlag the same as sleep debt?
No. Midpoint difference describes timing. Two eight-hour sleep schedules can have different midpoints even when their durations match.
Does this reveal my biological circadian phase?
No. Sleep timing is influenced by work, light, alarms and many other factors. Biological phase assessment requires more than clock times.
Does longer free-day sleep affect the result?
Yes. Catch-up sleep can move the midpoint. This tool reports the uncorrected measure and does not apply a sleep-debt correction or classify the result as healthy or unhealthy.