Sleep debt calculator

The calculator takes each night from bedtime to wake time, so a night that crosses midnight counts in full. Each night under your need adds its gap to the total, so a night one hour short adds one hour. A bedtime equal to the wake time counts as a night with no sleep.

Long nights are not taken off the total. The extra sleep is shown on its own, because the research below finds that a lie-in does not simply cancel a short night. Naps are left out, because the advice on how much sleep you need is about a regular night.

Bedtime spread is the gap between your earliest and latest bedtime that week, a simple measure of how steady your nights were. The National Heart, Lung, and Blood Institute (NHLBI), a US government health institute, suggests keeping your sleep schedule within about 1 hour from weekdays to weekends (the NHLBI), so the calculator calls a spread of up to an hour steady.

Sleep debt does not clear in one weekend. In a lab study, Depner and colleagues gave people 5-hour nights for a work week, then a free weekend to sleep as long as they liked (Depner and colleagues, 2019). They slept only about 1.1 hours extra in total, and once the short nights came back, their body clocks (the inner timing that makes you sleepy at night) ran later than before.

Recovery is slow even when there is plenty of time to sleep. Kitamura and colleagues let people sleep as long as they liked for days in a lab (Kitamura and colleagues, 2016). Just 1 hour of debt took 4 days to recover.

So the number worth watching is next week’s, not last week’s. To see what a steady shortfall adds up to, read is 6 hours of sleep enough.

Total hours are only half of what the calculator shows, because researchers also look at how steady sleep times are. Phillips and colleagues followed 61 college students who kept sleep diaries for 30 days (Phillips and colleagues, 2017). The students whose bed and wake times jumped around had body clocks running 2h 36m later than the students who kept regular hours, and their grades were lower.

A much larger study points the same way. Windred and colleagues looked at 60,977 UK adults who wore activity trackers, and regular sleep timing predicted the risk of early death better than sleep length did (Windred and colleagues, 2024). The study shows a link, not a cause.

A steady bedtime starts with a steady wake time. For a dated plan that moves yours, see how to fix your sleep schedule.

The calculator works from the times you remember, and memory tends to round sleep up. In one study, people who wore sleep trackers on the wrist said they slept 6.8 hours, while the trackers measured 6 (Lauderdale and colleagues, 2008).

A good way to track this counts your nights from what you already write or say each morning. No timers, nothing to wear. Most journaling apps stop at how the night felt, which never adds up to hours short.

One app built for this is Think in Numbers, a smart journaling app with time tracking built in.

You write or say your day in your own words, as it happens or all at once. The app reads what you wrote and counts the hours that went to each part of it: sleep, your phone in bed, and the time you never mentioned (shown as unaccounted). Then it shows those hours over days and weeks. A target holds each week against your need.

Write my first day

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For iPhone and Android. Try it on your own day before you pay.

Every figure on this page links to one of these.

1. Watson and colleagues (2015). Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med.
2. National Heart, Lung, and Blood Institute (read October 1, 2026). Healthy Sleep Habits.
3. Depner and colleagues (2019). Ad libitum weekend recovery sleep fails to prevent metabolic dysregulation during a repeating pattern of insufficient sleep and weekend recovery sleep. Curr Biol.
4. Kitamura and colleagues (2016). Estimating individual optimal sleep duration and potential sleep debt. Sci Rep.
5. Phillips and colleagues (2017). Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Sci Rep.
6. Windred and colleagues (2024). Sleep regularity is a stronger predictor of mortality risk than sleep duration: a prospective cohort study. Sleep.
7. Lauderdale and colleagues (2008). Self-reported and measured sleep duration: how similar are they? Epidemiology.