Sleep & recovery

Sleep debt explained: how to calculate it and pay it back

Updated 29 October 2026 · 4 min read

One short night rarely hurts your training. A week of them does. Sleep debt is the running total of the sleep you needed and didn’t get, and it explains a lot: the easy run that felt hard, the heavy legs on a rest week, the cold after a busy month. Here is how to work out yours, how quickly it comes back, and what it means for the session in front of you.

How to calculate your sleep debt

1. Find your sleep need. On a holiday without an alarm, after the first few catch-up nights, note how long you sleep. That is close to your need. Without that, start at 8 hours.
2. Take the last 14 nights of time asleep, not time in bed. The Health app shows it under Browse ▸ Sleep.
3. Subtract each night from your need and add up the shortfalls. Extra sleep on a night pays some of it back.
4. Weight recent nights more. Last week matters more to how you feel today than the week before. Many models let older nights count less, day by day.

An example: you need 8 hours and slept 6.5 hours on five work nights, then 8.5 on two weekend nights. That is 7.5 hours short, minus 1 hour paid back: 6.5 hours of debt from one week. Our sleep debt calculator does the sum for you, recent nights weighted more.

Paying it back

Sleep debt comes back slowly, and not hour for hour. A couple of hours of debt can clear in a night or two of longer sleep. A large debt built over weeks takes longer, often several nights of 30–60 minutes extra. What helps most:

• Go to bed earlier rather than sleeping in. A late wake-up shifts your body clock and makes the next night harder.
• Keep bed and wake times regular, within about an hour, weekends included.
• Nap early and short. 20–30 minutes before mid-afternoon eases the debt without spoiling the night.
• Protect the hours before bed: no hard sessions late in the evening, less alcohol and caffeine, a cool, dark room.

What sleep debt means for training

Short sleep raises how hard a given effort feels and slows recovery between sessions. It also shows up in the numbers your watch records: resting heart rate drifts up and HRV down after several short nights. That is why a readiness score that leaves out sleep misses half the picture. Practical rules many coaches use:

• With a high debt, swap an interval session for an easy run or a rest day rather than forcing it.
• Easy, short training is usually fine and can help you sleep.
• Before a race, the week matters more than the night. A poor last night before a well-slept week costs little.

Sleep debt in Keho

Keho reads your nights from Apple Health and shows your sleep debt on the Sleep page, with sleep stages and bedtime regularity beside it. Naps from your Apple Watch, or found in Polar’s 24/7 data, appear beside the night. They never count towards your sleep goal, but they can ease your sleep debt. Your readiness score each morning weighs your sleep alongside HRV, resting heart rate and training load, and if you follow a Keho Coach plan, a tough stretch can bring an easier option for the day. You decide. To see where your stages come from, read how to see your sleep stages on iPhone.

Common questions

Can I catch up on sleep at the weekend?

Partly. A longer night pays some debt back, but it doesn’t undo a week of short nights, and a much later wake-up makes Sunday night harder. Earlier bedtimes through the week work better.

How many hours of sleep do athletes need?

Most adults need 7–9 hours. People in hard training often need the top of that range or more. Your own need is the length you settle at when you sleep without an alarm.

Is sleep debt a medical measure?

No. It is a useful training guide, not a diagnosis. If you sleep poorly for weeks, snore heavily or feel sleepy during the day despite enough time in bed, talk to a doctor.