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The formula in one line
Sleep efficiency is the share of your time in bed that you spend asleep. You take the time you slept, divide it by the time you were in bed, and multiply by 100.
Time in bed starts when you settle down to sleep and ends when you get up for the day. Time asleep is your best guess at how much of that you slept. Both come from a sleep diary, filled in each morning.
A worked example from one night
Say your diary for Tuesday night reads like this:
- In bed at 11pm, up at 7am. That is 8 hours, or 480 minutes, in bed.
- It took about 45 minutes to fall asleep.
- You woke at 3am and lay awake for about an hour.
- You woke for good at 6:30am and stayed in bed until 7am.
Add up the awake time: 45 + 60 + 30 = 135 minutes. Take that from 480 and you get 345 minutes asleep, which is 5 hours 45 minutes.
Now divide: 345 ÷ 480 = 0.72. Multiply by 100 and the night's sleep efficiency is 72 per cent. If you would rather not do the sum by hand, the sleep efficiency calculator does it for you.
Why a weekly average matters more than one night
Single nights jump around. A late dinner, a noisy street or a work worry can pull one night down. That says little about your usual pattern.
So people tend to add up a full week. You total the time asleep across seven nights, total the time in bed, and divide one by the other. That weekly figure is steadier and much more useful than any single night.
Your estimates do not need to be exact. Guessing to the nearest 15 minutes is fine. Clock-watching to get precise numbers tends to keep people awake, so look at the clock as little as you can.
How the behavioural approach uses it
In cognitive behavioural therapy for insomnia (CBT-I), the weekly figure helps set the sleep window, which is the stretch of night you plan to spend in bed. The AASM guideline describes this kind of diary-based approach.
The general idea looks like this:
- The window starts close to the amount of sleep you are getting.
- When the diary shows mostly solid nights, the window may grow a little.
- When nights are mixed, it stays where it is.
- When nights are mostly broken, it may shrink a little, but never below a safety floor.
In a self-guided programme, an app can suggest a change from this rule, and you decide whether to make it. Shortening time in bed can leave you sleepy in the day. It sits behind a safety check, and some people need to speak to a clinician first. That includes anyone with bipolar disorder, epilepsy, suspected sleep apnoea or a job where drowsiness puts people at risk, and anyone who is pregnant. The sleep window explainer goes through it step by step.
Why a wearable's number is a different thing
Many watches and rings show a figure they call sleep efficiency. It comes from movement and heart rate, so the device is guessing when you were asleep. Studies of consumer devices find they often miss time spent lying awake and still.
The diary version measures something else: how the night felt to you. That is the measure the behavioural approach is built on. There is more on this in why a diary beats a wearable.
Where the definitions differ
Researchers do not all divide by the same thing. Some use time in bed. Others use the time from first trying to sleep to final waking, which leaves out lying in afterwards. Reed and Sacco (2016) set out how much this choice can change the answer.
The Consensus Sleep Diary was written to help researchers record nights the same way. The point for you is to pick one method and stick with it, so this week can be compared with last week.
This is general education and is not a plan for you. If your nights stay short and your days feel unsafe, talk to a GP. A week of diary entries is a good thing to bring along.
FAQ
- How do you calculate sleep efficiency?
- Divide the time you spent asleep by the time you spent in bed, then multiply by 100. Both numbers come from your sleep diary, so they are your own best estimates.
- What is a good sleep efficiency?
- There is no single figure that counts as good, and it is not a measure of how healthy your sleep is. It is most useful for spotting patterns across a week, because one night tells you very little on its own.
- Why is my sleep efficiency low?
- Usually because you are spending a lot of time in bed awake: taking a long time to drop off, waking in the night, or lying in after waking. A clinician can help you look at causes if it stays low.