How to Calculate Sleep Debt — Step by Step

The six steps at a glance

Sleep debt is the gap between the sleep you need and the sleep you get. The week needs seven nights of your target, and the week gives you five weeknights and two weekend nights. The six steps are: pick your target, work out the weekly need, count the weeknight sleep, count the weekend sleep, add them up and subtract, and work out the recovery. The worked example comes to 5.50 hours of debt.

Step 1 — Pick your target

Pick how many hours of sleep a night you aim for. About 8 hours is typical for an adult, and that is the target the worked example uses. A target of 7 hours is common too, and a target of 9 hours is common for teenagers and for people catching up after illness. Write the number down: it is the one number every later step uses, so a target that is wrong by an hour makes the debt wrong by 7 hours a week.

Step 2 — Work out the weekly need

The weekly need is seven times your target, because the week has seven nights. For the worked example, 7 times 8 is 56.00 hours of sleep a week. That is the sleep the week needs before a single night is counted, and it is the number the debt is measured against at the end.

Step 3 — Count the weeknight sleep

The weeknight sleep is five times the sleep you get on a weeknight, Sunday to Thursday. For the worked example, 5 times 6.5 is 32.50 hours across the five weeknights. Count the sleep you actually get, not the time you spend in bed: if you go to bed at 11 and fall asleep at 11.30, count from 11.30.

Step 4 — Count the weekend sleep

The weekend sleep is two times the sleep you get on a weekend night, Friday and Saturday. For the worked example, 2 times 9 is 18.00 hours across the two weekend nights. The weekend is only two nights, so it counts less than the weeknights, but each weekend night counts double in the total, because it is one of only two. Count the weekend the same way as the weeknights: the sleep you actually get, not the time in bed.

Step 5 — Add them up and subtract

Add the weeknight sleep and the weekend sleep together: for the worked example, 32.50 plus 18.00 is 50.50 hours of sleep a week. That is the weekly actual. Then subtract it from the weekly need: 56.00 minus 50.50 is 5.50 hours, and that is the sleep debt. A positive number is hours owed, and a negative number is a surplus, not a debt: with a target of 7 hours the need is 49.00 hours while the actual is 50.50 hours, so the debt is minus 1.50 hours, which is a 1.50-hour surplus. The average night is the weekly actual divided by 7: 50.50 divided by 7 is 7.21 hours a night.

Step 6 — Work out the recovery

The recovery is the debt divided by the extra sleep a night. For the worked example, 5.50 divided by 1 is 5.50 nights at one extra hour a night, and 5.50 divided by 2 is 2.75 nights at two extra hours a night. One extra hour a night is the usual plan, because it fits into a normal evening. The break-even weeknight sleep is 7.60 hours: 5 times 7.60 plus 18.00 is 56.00 hours. The break-even weekend sleep is 11.75 hours.

The worked example, line by line

Setup: target 8 hours a night, weeknight 6.5 hours, weekend 9 hours, extra 1 hour a night. Line 1, the weekly need: 7 times 8 is 56.00 hours. Line 2, the weeknight sleep: 5 times 6.5 is 32.50 hours. Line 3, the weekend sleep: 2 times 9 is 18.00 hours. Line 4, the weekly actual and the debt: 32.50 plus 18.00 is 50.50 hours, and 56.00 minus 50.50 is 5.50 hours of debt. Line 5, the average night and the recovery: 50.50 divided by 7 is 7.21 hours a night, and 5.50 divided by 1 is 5.50 nights at one extra hour a night, or 2.75 nights at two extra hours a night. Every figure here comes from the same six steps, and the sleep debt calculator runs them for any sleep you enter.

Other weeks, checked

A weeknight of 7 hours gives a weekly actual of 53.00 hours and a debt of 3.00 hours. A weekend of 7 hours gives a weekly actual of 46.50 hours and a debt of 9.50 hours. A target of 7 hours gives a weekly need of 49.00 hours and a 1.50-hour surplus. A target of 9 hours gives a weekly need of 63.00 hours and a debt of 12.50 hours. A weeknight of 5 hours gives a weekly actual of 43.00 hours and a debt of 13.00 hours. The same pattern for 4 weeks is 22.00 hours owed, or about 2.75 full nights of sleep.

What the six steps leave out

The six steps count hours, and they leave out sleep quality: a long night of broken sleep is not the same as a long night of deep sleep, and the six steps cannot tell the difference. Naps are not counted, because the six steps count night sleep only. The six steps assume the same sleep every weeknight and the same sleep every weekend night. This is a simple hours model, not a medical assessment: if you are worried about your sleep, talk to a clinician. For your sleep timing, see the sleep cycle calculator, and for a general health check, see the BMI calculator.

Case	Need	Actual	Debt	Average night
Default (8 h target)	56.00 h	50.50 h	5.50 h	7.21 h
Weeknight 7 h	56.00 h	53.00 h	3.00 h	7.57 h
Weekend 7 h	56.00 h	46.50 h	9.50 h	6.64 h
Target 7 h	49.00 h	50.50 h	-1.50 h (surplus)	7.21 h
Target 9 h	63.00 h	50.50 h	12.50 h	7.21 h
Weeknight 5 h	56.00 h	43.00 h	13.00 h	6.14 h