Sleep
Sleep trackers measure something, just not always sleep
Wearables are useful for spotting trends and unhelpful for judging a single night. Knowing which is which prevents a specific kind of anxiety.
Everything here earned its place by changing an outcome. Nothing about sleep tracking is included to round the number up.
What matters most
- Consumer devices infer sleep stages from movement and heart rate, and the inference is approximate.
- Trends over weeks are informative; single-night stage breakdowns are not.
- Anxiety about tracker scores is a documented phenomenon with a name.
What the device actually measures
A wrist wearable records movement and, usually, heart rate and heart rate variability. From those signals it estimates whether you were asleep and, more speculatively, which stage you were in. The gold standard is polysomnography, which measures brain activity directly, and agreement between the two on staging is moderate at best.
Total sleep time is estimated reasonably well; the coloured stage chart is the least reliable part of the display.
Trends are where the value is
Averaged across weeks, these devices are quite good at showing that your sleep got shorter, more fragmented or later. That is genuinely useful, because those trends correlate with things you can change — schedule, alcohol, stress, exercise timing. Reading a single night's breakdown and concluding something about your deep sleep is reading noise.
The algorithms also change with firmware updates, so a step in your averages sometimes reflects a software release rather than anything that happened to you.
Orthosomnia is a real problem
Clinicians have described a pattern where people become anxious about optimising tracker scores and sleep worse as a result. It follows directly from checking the score on waking and letting it set the expectation for the day.
The useful part is this: if the number affects your mood more than the night did, the device has stopped being useful. The effect appears strongest in people already anxious about sleeping, which is also the group most likely to have bought a tracker in the first place.
What trackers are genuinely good at
Detecting irregular schedules is their strongest use, because timing is easy to measure and matters a lot. Some devices flag unusually low overnight oxygen saturation or high resting heart rate, which can prompt a useful medical conversation. They are also good at making an invisible behaviour visible, which is often enough to change it.
Those alerts are prompts rather than findings, and a device hinting at apnoea or an irregular rhythm is a reason to see a doctor rather than a conclusion in itself.
How to use one without it using you
Look at weekly averages, not nightly scores, and ideally not first thing in the morning. Pick one metric — usually total sleep time or bedtime consistency — and ignore the rest. Take a break from wearing it periodically, and notice whether your sleep or only your data changes.
Turning off the notification that pushes the score at you on waking removes most of the harm while keeping all of the data.
What it cannot replace
No consumer device diagnoses a sleep disorder, and none should be used to rule one out. If you snore heavily, wake unrefreshed, or fall asleep during the day, that is a clinical question regardless of what the app says. A tracker can be the reason you make the appointment, which is a good use of it.
The useful part is this: proper assessment is also done at home rather than in a hospital in many places now, so the alternative to a wearable is less daunting than people assume.
Everything above, in order of what to do first
- What the device actually measures. A wrist wearable records movement and, usually, heart rate and heart rate variability.
- Trends are where the value is. Averaged across weeks, these devices are quite good at showing that your sleep got shorter, more fragmented or later.
- Orthosomnia is a real problem. Clinicians have described a pattern where people become anxious about optimising tracker scores and sleep worse as a result.
- What trackers are genuinely good at. Detecting irregular schedules is their strongest use, because timing is easy to measure and matters a lot.
- How to use one without it using you. Look at weekly averages, not nightly scores, and ideally not first thing in the morning.
- What it cannot replace. No consumer device diagnoses a sleep disorder, and none should be used to rule one out.
The takeaway
Read the week, not the night, and stop reading it before breakfast.
Small and repeatable beats ambitious and abandoned, almost every time.
Questions readers ask
Are sleep stage percentages accurate?
Not particularly. Consumer devices agree with clinical measurement reasonably on sleep versus wake and much less on which stage you were in.
Should I stop using my tracker?
Only if it is making you anxious. Otherwise, use it for weekly trends and ignore the nightly score.
