What sleep trackers get right, and what they guess

Millions of people now wake up and check a sleep score before they check the weather. Trackers report total sleep, deep sleep, dream sleep and something called readiness. Some of those numbers are well measured. Others are educated guesses dressed up as facts.

What they measure well

Wrist and ring trackers are good at telling when you are asleep versus awake and moving. Over many nights, their estimate of total sleep time tends to be within about half an hour of a sleep lab’s measurement.

Your tracker is very good at knowing you were still. It is guessing about why.

What they estimate

• Sleep stages such as deep and dream sleep, which are properly measured with brain sensors.

• Exact times of falling asleep, especially if you lie still while awake.

• Readiness or recovery scores, which blend several estimates into one number.

• Anything to do with the quality of your dreams, despite what the app suggests.

How to use one sensibly

Look at trends across weeks rather than single nights. If your total sleep is steadily lower than you would like, that is useful information. If last night’s deep sleep figure was low, it may simply be wrong.

And if checking the score makes you anxious, which some sleep researchers report is common, it is fine to take the tracker off for a while. You will still sleep.

Signs worth taking seriously

Trackers can be genuinely helpful for spotting patterns you would otherwise miss. A steady rise in resting heart rate over several nights can be an early sign of illness or overtraining. Consistently short sleep on work nights is useful to see in black and white.

What a tracker cannot do is diagnose a sleep problem. Loud snoring with pauses in breathing, waking exhausted despite long nights, or falling asleep during the day are reasons to speak to a doctor, whatever the app says about your score.

Used well, a sleep tracker is a diary that fills itself in. Used badly, it is one more thing to worry about at 3am. The difference is mostly whether you look at the week or the night.