Consumer sleep trackers are remarkable pieces of engineering. They are also not measuring sleep directly. Understanding what they actually sense makes their numbers much easier to use — and much harder to obsess over.
The sensors
- Accelerometer. Movement. Most “sleep” is inferred from stillness plus time of day. A restless night with little movement can look peaceful; lying still and awake can look like sleep.
- Photoplethysmography (PPG). The green or infrared light on the back of a watch or inside a ring. It estimates heart rate and, from the variation between beats, heart rate variability.
- Temperature. Skin temperature changes with sleep and with the room. Useful as a trend, noisy as a single night.
- Respiration and oxygen. Some devices estimate breathing rate and blood oxygen. The latter is not a medical oximeter and should not be treated as one.
From those signals, an algorithm guesses sleep stages. The guess is compared, in validation studies, with polysomnography — the laboratory standard with electrodes on the scalp, face and legs. The agreement is usually decent for total sleep time and poor for the fine detail of stages.
Where they disagree
Two devices on the same wrist can report different sleep times by an hour or more. A ring may call a quiet reading session “light sleep”; a phone on the nightstand may miss half the night because you moved it. None of this means the devices are broken. It means they are estimating different things from different signals.
A tracker is a weather vane, not a diagnosis. It tells you which way the wind is blowing, not why.
What they are good for
- Trends over weeks. Is your sleep duration drifting down? Is your resting heart rate rising? Trends survive the noise better than single nights.
- Behaviour change. Seeing a bedtime creep later is often more useful than any stage breakdown.
- Starting a conversation. A month of data is a useful thing to bring to a doctor — as context, not as proof.
The trap
There is a name for the anxiety that comes from watching your sleep score: orthosomnia. Chasing a perfect number can make sleep worse, because the watching itself is arousing. If the tracker makes you dread bedtime, the tracker is the problem.
Use it for a month, learn your pattern, then consider putting it in a drawer for a week. The sleep diary on this site exists for exactly that reason: it is boring, cheap, and it does not glow at 3 a.m.