Sleep trackers
Consumer sleep trackers — watches, rings and bedside devices — estimate your sleep stages from movement and heart rate rather than from brainwaves. They are reasonable for bedtime consistency and roughly how long you slept, and poor at the stage breakdown: the best miss about a third of true deep sleep, and different brands err in opposite directions. Chasing their scores may have worsened insomnia in a few reported patients, though whether trackers cause that is barely tested: the one small controlled comparison found no difference.
Use a tracker, if you like one, for bedtime consistency and total sleep. Ignore the deep-sleep and REM figures, and don't judge a supplement, gadget or habit by what it does to them.
How they work, and what they copy
The reference for sleep staging is polysomnography, a lab sleep study that records brainwaves (electroencephalogram, EEG), scored by a technician in 30-second windows. A tracker has none of those signals. It infers stages from wrist or finger movement and pulse, using an algorithm trained to reproduce technician-scored labels, as Fitbit's own published description of its algorithm sets out.[1] So even a well-validated tracker inherits the weaknesses of the scoring rules it copies, including the fixed brainwave-height threshold for deep sleep that the Sleep architecture article explains.
Deep sleep: wrong, and wrong in different directions
Most devices miss much of it. Across the six of seven consumer devices that stage sleep, tested against a lab sleep study, the best missed a third of true deep-sleep periods and the worst nearly half.[2]
Newer hardware hasn't fixed it, and the error depends on the brand. A 2025 test of six current wrist devices in 62 adults recruited at a hospital sleep clinic, mostly men and over half with sleep apnea, average age 46, found:[3]
| Device | Deep-sleep error vs. lab sleep study |
|---|---|
| Garmin Vivosmart 4 | +44 minutes (overestimates) |
| Whoop 4.0 | +31 minutes (overestimates) |
| Fitbit Charge 5 / Sense | ~2–4 minutes low (not significant) |
| Apple Watch Series 8 | −25 minutes (underestimates) |
A sixth, the Withings Scanwatch, was further out still at +73 minutes, but it lumps deep and dreaming sleep together, so it isn't comparing like with like. These are group averages across different subsets of the 62 people, not one person wearing two watches, but the spread is the point: the brand you buy moves the number more than anything you do at night.
Rings do no better. In 45 sleep-clinic patients, the best ring tested, the Oura, caught about half of true deep-sleep periods and another caught under a fifth, even though the Oura's average deep-sleep total was close to right.[4] A near-correct average can hide a device that is wrong night by night.
Total sleep: better, but less certain in older adults
Trackers track bedtime consistency and total sleep reasonably well. In older adults that is less certain. In one small, one-night study of 19 people aged 56–80, all four devices tested read total sleep low, the Fitbit and Oura by about 75 minutes, and three of the four read deep sleep high.[5] Other studies have found devices reading total sleep high,[6] so treat this as a warning rather than a settled answer.
Even the lab number wobbles
The target the trackers aim at is itself unsteady. Put the same older adult in a sleep lab two nights running, with a human scoring both, and deep-sleep percentage is only moderately reproducible; the underlying continuous brainwave signal is steadier.[7]
Orthosomnia: when the score becomes the problem
Anxiously chasing sleep scores has a name, orthosomnia, coined in a report of three patients whose pursuit of perfect tracker numbers, the authors suggest, may have worsened their insomnia.[8] That report had no comparison group. The one small controlled test gave 14 primary-care insomnia patients a Fitbit (plus a self-help booklet) and 12 a paper sleep diary for four weeks; both groups improved on sleep quality, with no difference between them.[9] That is too small to rule out harm, but it is the only test of whether trackers cause the problem, and it didn't find it. If checking your score makes you anxious about sleep, stop checking it.
What to do
- Use the tracker for timing and total sleep, and keep your schedule regular.
- Ignore the stage breakdown. A higher deep-sleep figure after a supplement, gadget or drink is not evidence that anything improved; alcohol can raise the number on a worse night.
- Don't compare scores across brands, or across a device change.
- If the numbers are making you anxious, take the tracker off. For persistent insomnia, see Treating chronic insomnia.
Further reading
- Chinoy ED, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep 2021.[10]
- Schyvens AM, et al. A performance validation of six commercial wrist-worn wearable sleep-tracking devices for sleep stage scoring compared to polysomnography. Sleep Adv 2025 — n=62, recruited from a hospital sleep clinic.[11]
- Herberger S, et al. Performance of wearable finger ring trackers for diagnostic sleep measurement in the clinical context. Sci Rep 2025.[12]
- Searles ME, et al. Performance evaluation of consumer sleep-tracking wearables and nearables in healthy young and older adults. Sleep Adv 2026.[13]
- Baron KG, et al. Orthosomnia: are some patients taking the quantified self too far? J Clin Sleep Med 2017.[14]
- Ojalvo D, Pacheco AP, Benedict C. A useful tool or a new challenge? Hand-wrist-worn sleep trackers in patients with insomnia. J Sleep Res 2023.[15]