Social Jet Lag Calculator

How it is calculated

For each kind of day the tool finds the sleep duration and its midpoint. With times as minutes after midnight, duration = (wake − bed) mod 1440, so sleep that crosses midnight works, and midpoint = bed + duration ÷ 2, wrapped to a 24-hour clock. Social jet lag is |free-day midpoint − work-day midpoint|, taking the shorter way round the clock (never more than 12 hours). It is shown in hours and minutes and labelled by size: under 60 minutes is low, 60 to 120 minutes moderate, over 120 minutes high. Times must be typed as HH:MM on a 24-hour clock. A night with bedtime equal to wake time, or a sleep longer than 18 hours, is not treated as a real night and gives no result. No correction is made for sleep debt or alarm clocks.

Common mistakes

• Typing times with a dot or in am/pm form. The fields need a 24-hour HH:MM value such as 23:00 or 07:30; 11pm gives no result.
• Entering bed and wake times the wrong way round. A bedtime of 07:00 and a wake time of 23:00 describes a 16-hour sleep and puts the midpoint at 15:00, giving a nonsense gap of 9h 30m; beyond 18 hours there is no result at all.
• Using a single unusually late weekend night. The tool compares typical days, so enter what you do most weeks, not the exception.
• Comparing the bed times or the wake times alone. The measure uses the midpoint of the whole sleep, so a later bedtime offset by a later wake time shifts the midpoint only by half the difference.
• Taking a free-day sleep-in as proof of social jet lag when your bedtime is also later. The tool counts both in the midpoint and does not separate them.