Put two sleep trackers on the same sleeper and they will mostly agree on one thing: roughly when you fell asleep and when you woke up. Ask them what happened in between and the answers drift apart.
That is the pattern in a Terra Research comparison of devices across 2,144 overlapping nights. Total sleep time agreed within about 20 minutes at the median. Sleep-stage minutes, the deep, light and REM breakdown most apps lead with, differed by more than 250 minutes on average. Terra Research (opens in a new tab)
What you can lean on
Total sleep time and the timing of sleep are the numbers that line up best across devices, which makes them the most useful for spotting trends over weeks. Sleep Foundation's advice runs the same way: use a tracker to find patterns, and weigh them against how rested you actually feel. Sleep Foundation (opens in a new tab)
What to hold loosely
Stage minutes are an estimate from each company's algorithm. A big swing in "deep sleep" from one night to the next may say more about the model than about you. Wirecutter, which tests trackers against a heart-rate sensor and sleep diaries, also found its top pick often counted reading or watching TV as a nap. Wirecutter (opens in a new tab)
What no tracker can do
Diagnose you. Wirecutter states plainly that no commercially available sleep tracker can perfectly analyse your sleep or diagnose a sleep disorder. Wirecutter (opens in a new tab) Apple, for instance, says its Sleep Score is not a diagnostic for sleep apnea Apple Watch User Guide (opens in a new tab), while its separate, FDA-cleared apnea notification feature only flags possible signs in adults Apple Support (opens in a new tab). If you are worried about your sleep, see a clinician.
When tracking backfires
For some people, chasing a better score makes sleep worse. Wirecutter cites a 2017 Journal of Clinical Sleep Medicine article describing "orthosomnia", a perfectionist quest for ideal sleep data Wirecutter (opens in a new tab), and Sleep Foundation suggests people who get anxious about health numbers skip trackers altogether Sleep Foundation (opens in a new tab).
How this shapes our scores
It is why accuracy, the largest part of our scorecard at 25 of 100 points, is judged mainly on total sleep time and wake detection against a sleep diary and a second device, with stage accuracy weighted lightly. When we test, we will publish the nightly numbers so you can check our work.
Questions
Are sleep trackers accurate?
For total sleep time, trackers tend to agree with each other closely: a Terra Research comparison of 2,144 overlapping nights found a median gap of about 20 minutes. For sleep stages they disagree a lot, by more than 250 minutes on average in the same comparison.
Can a sleep tracker diagnose sleep apnea or insomnia?
No. Wirecutter notes that no commercially available tracker can diagnose a sleep disorder. Apple, for example, says its Sleep Score is not a diagnostic for sleep apnea; its separate FDA-cleared notification feature flags possible signs and is not a diagnosis.
Can tracking make sleep worse?
It can for some people. Wirecutter points to a 2017 Journal of Clinical Sleep Medicine article describing "orthosomnia", a perfectionist pursuit of ideal sleep data. Sleep Foundation advises people who get anxious about health numbers to avoid trackers.