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Sleep debt calculator: how much sleep you owe
Sleep debt is the sleep you needed but didn’t get, added up over the last couple of weeks. One short night barely registers; a week of them does. Enter how much sleep you need and what you actually slept on recent nights, and the calculator shows your debt, weighting recent nights more, and roughly how long it would take to sleep it off.
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Short answer
Sleep debt is the sum of each night’s shortfall against your sleep need over the last 14 nights. Needing 8 hours and sleeping 7 every night adds up to 14 hours short; with recent nights counted more, as here, about 10.5 hours. You pay it back gradually with longer nights and earlier bedtimes, not with one long lie-in.
How the calculation works
For every night, the shortfall is your sleep need minus what you slept, and never less than zero: a long night doesn’t create credit. The calculator then weights the nights, because last week’s short sleep affects you more today than the week before. Last night counts in full, and the weight falls in equal steps to one half for the night two weeks ago. Below the weighted debt you also see the plain total. Nights you leave empty are skipped, not counted as zero.
The payback estimate assumes one extra hour of sleep a night, which is about as much as most people can add without lying awake. It is a rough guide: research suggests that recovering from a long stretch of short sleep takes more than adding back the missing hours, and some of the effects on attention linger for days.
Finding your sleep need
Sleep need differs between people. A practical test: on a holiday without an alarm, after the first few nights of catching up, note how long you sleep on average. That is close to your need. Athletes in heavy training often need more than their usual amount, and recovery suffers first when they don’t get it. Read more in sleep debt explained.
Sleep debt and training
Short sleep shows up in the numbers before you feel it: a higher resting heart rate, a lower HRV and hard sessions that feel harder than they should. When debt builds, an easier day or an earlier night usually gets you more than a hard session would. Keho tracks your sleep debt from the nights in Apple Health automatically, counts naps that ease it, and uses it in the morning readiness verdict next to HRV, resting heart rate and training load.
Questions
Do naps count?
Yes, a nap reduces the shortfall for that day, though it is no full substitute for night sleep. Add nap time to the night before if you want to include it here.
Should I count time in bed or time asleep?
Time asleep. Most people lie awake for 10 to 30 minutes a night in total, so time in bed overstates sleep. Your watch’s sleep stages in Apple Health show time asleep; see sleep stages on iPhone.
Is this medical advice?
No. If you regularly sleep badly, wake up unrefreshed or snore heavily, talk to a doctor.