Sleep efficiency: what it is and why 85% matters

Sleep efficiency is the share of your time in bed that you spend asleep. Divide the time you slept by the time from going to bed until getting up, and multiply by 100. In insomnia treatment, about 85% or more is the level usually worked towards.

How it's worked out

The American Academy of Sleep Medicine's insomnia guideline defines it from a sleep diary:

• Time in bed: from going to bed until getting out of bed.
• Total sleep time: time in bed, minus the time it took to fall asleep, minus the time awake after first falling asleep.
• Sleep efficiency: total sleep time divided by time in bed, times 100.

For example, in bed from 22:45 to 06:50 is 8h 5m. Asleep for 7h 15m of that gives 90%. The sleep efficiency calculator does the sum.

Researchers don't all divide by the same thing. Some divide by the time spent trying to sleep rather than the whole time in bed, which gives a higher number if you read in bed first. The time-in-bed version is the widely published definition, and the one the calculator and the app use.

What's a good number

In sleep restriction therapy, part of cognitive behavioural therapy for insomnia (CBT-I), people usually keep reducing their time in bed while efficiency stays below 85%, and hold their schedule steady once it's between 85% and 89%. That's why 85% comes up so often.

It's a working threshold for treatment, not a diagnosis. One night under 85% tells you little on its own. What matters is the average over a week or two, and whether the low nights have something in common.

What a low number means

A lot of time in bed awake: taking a long time to fall asleep, waking in the night, or waking early and staying in bed. Your diary shows which. Long times to fall asleep, and long spells awake in the night, are both things a doctor will ask about.

Don't chase the number on your own

Cutting your time in bed raises efficiency, and it's part of a recommended treatment, but it's done with a clinician who sets and adjusts the window from your diary. The NHS suggests changing sleep habits first, and seeing a GP if that hasn't helped or poor sleep has gone on for months; a GP may offer CBT for insomnia.

This guide isn't medical advice. If poor sleep has gone on for months or makes the day hard to cope with, see your doctor.