Sleep Trackers: Trust the Trend, Not the Stage Breakdown

Summary

A consumer sleep tracker is a **consistency-and-timing instrument, not a sleep lab** — it knows reasonably well *when* you slept (sleep/wake detection, total sleep time, bedtime regularity are decent), but it is largely **guessing at how** (the deep/REM/light pie chart shows only fair-to-moderate agreement with the clinical gold standard, and devices systematically over-count sleep by reading quiet wakefulness as sleep) — so watch the multi-week direction of total sleep and bedtime regularity, treat the nightly stage breakdown as decorative, and recognise that fixating on a perfect "sleep scor