How accurate are sleep trackers for REM and deep sleep?

Sleep trackers are best at the simplest question: were you asleep or awake? That distinction is driven largely by movement and is reasonably reliable. The impressive-looking part of the report — the breakdown of REM versus deep versus light sleep — is where they are weakest. Consumer wearables infer sleep stages from indirect body signals rather than measuring brain activity directly, which is what a clinical sleep study (polysomnography) does with EEG. On Rhonda Patrick's show, sleep researcher Dr. Michael Grandner describes the stage breakdown as a rough estimate, not a precise nightly measurement.

A second caution is the "sleep score" or "readiness" number. Grandner's point is that these are often proprietary and not clinically validated — they do not necessarily map onto any physiological outcome — so treating one number as a verdict on your health can mislead. The more useful move is to watch consistent, long-term trends: your typical total sleep time, how regular your schedule is, and signals like resting heart rate, rather than obsessing over one night's stage percentages.

Used this way, a tracker can still help. It can reassure you that you slept more than it felt like, or surface a pattern of fragmented sleep that explains daytime fatigue. And if the data shows persistently irregular patterns, that can be a nudge to look at your environment or to talk to a clinician about a possible issue like sleep apnea. What it cannot do is diagnose you — the stage percentages are a trend tool, not a medical measurement.