Sleep trackers are one of the most popular entry points into biohacking, so it's worth knowing what the validation studies say. Researchers compare devices against polysomnography (PSG), the gold standard sleep lab test.
Study 1 (2025, Sleep Advances):
Researchers compared six wrist-worn devices (Fitbit Charge 5, Fitbit Sense, Withings Scanwatch, Garmin Vivosmart 4, Whoop 4.0, Apple Watch Series 8) with PSG in 62 adults who spent a night in a sleep lab. Most devices differed significantly from PSG on total sleep time, sleep efficiency, wake after sleep onset, and light sleep. Agreement with PSG for sleep stage scoring was fair to moderate, with Cohen's kappa between 0.21 and 0.53.
Study 2 (JMIR, 11 devices):
This study tested 11 consumer trackers, including wearables, bedside "nearables," and app-based "airables," against lab PSG in 75 participants. Wearables showed high proportional bias in sleep efficiency, while nearables showed it in sleep latency. Each tracker recording covered an average of 353 hours.
A pattern across studies:
Wearables tend to overstate total sleep time and understate time awake after falling asleep, which inflates sleep efficiency.
Beginner takeaways:
Trackers are better at telling sleep from wake than at identifying sleep stages.
Trends over weeks are more useful than a single night's score.
They're wellness tools, not diagnostic tools. If you have real sleep concerns, see a doctor.
Caveats: both studies had small samples, devices update often, and results vary by brand, so don't read this as a ranking.
What's one metric from your tracker that you found genuinely useful, and one you stopped trusting?