How accurate is your sleep tracker, and how should you use the numbers?

How does a sleep tracker know you're asleep?

A watch or ring doesn't “see” your brain. It looks at movement, using an accelerometer, and at your pulse, using a light that shines through the skin. Some also measure skin temperature or blood oxygen. From all of this, software estimates whether you're asleep and which stage of sleep you're in.

In a sleep lab, the full test is called polysomnography. It measures brain activity, eye movements and muscle tone directly. Sleep stages are defined from those signals, which is why a device on your wrist can only approximate them.

What does research say about accuracy?

In 2021, the journal Sleep published a study in which 34 young adults spent three nights in a lab wearing several devices at once: bands, watches and bedside sensors. The results were compared with polysomnography.

• They recognised sleep very well. When someone was asleep, the devices almost always said so.
• They found it harder to catch time awake. When someone was awake but lying still, many devices thought they were asleep.
• Sleep stages came out inconsistently, differently from one device to another.
• On restless nights, with many wake-ups, they all did worse.

The authors concluded that many newer devices look promising for tracking sleep and wake, but that they need testing in other groups too, not just young, healthy adults.

What does this mean for you?

Your tracker tells you fairly well when you fell asleep, when you woke up and roughly how long you slept. It's less precise about how long you lay awake at 3 am, and you can hardly rely on it for how many minutes of deep sleep you had.

So there's no point worrying about one night with “little deep sleep” or a low score. Sometimes worrying about the numbers spoils sleep in itself. Researchers have even given a name to people sleeping worse because they're preoccupied with their sleep data: orthosomnia.

To put the numbers in context, the daily journal in your free MentorSano account also asks how you feel in the morning, and after a few weeks you can see whether what your tracker shows matches your energy.

How do you use the numbers so they help?

Number	How far you can trust it	How to use it
Bedtime and wake-up time	fairly well	see whether you keep a regular schedule, weekends included
Total sleep time	fairly well, especially as an average	follow the 7-day average, not one night
Resting heart rate overnight	usually well	notice if it rises several days in a row, for example after alcohol, illness or stress
Wake-ups during the night	less so	look at the trend, not the number
Deep and REM sleep	unreliable	don't make decisions based on them
Sleep score	varies by device	compare yourself only with yourself, on the same device

What can you learn from trends?

The real value of a sleep tracker shows up after a few weeks. That's when you can see things you'd otherwise miss:

• that you sleep an hour less on weekdays than you think;
• that on evenings with alcohol your night-time heart rate is higher and your sleep shorter;
• that on days you move more, you fall asleep faster;
• that your bedtime drifts later and later towards the end of the week.

You see all of this from trends, not from a single night.

When isn't a tracker enough?

Consumer devices don't diagnose anything. They can't tell you on their own whether you have sleep apnoea or insomnia. If you snore loudly, wake up gasping or are very sleepy during the day, you need a consultation and sometimes a sleep recording arranged by a doctor. Some watches give oxygen or breathing alerts, but an alert is just a reason to see a doctor.