Sleep Tracking: What Wearables Can and Cannot Tell You
The Appeal and the Limits
Consumer sleep trackers -- Oura, Whoop, Apple Watch, Garmin -- have driven enormous interest in sleep monitoring. They provide data that were previously inaccessible outside a sleep lab, and the data can be genuinely useful. But understanding what the devices are actually measuring -- and where they reliably fail -- prevents both over-reliance and dismissal of useful signals.
What Trackers Measure Well
- Sleep duration: consumer wearables measure total sleep time with reasonable accuracy (within 10-20 minutes for most devices, compared to polysomnography)
- Sleep timing: when you fell asleep and when you woke is reliably captured
- Heart rate and HRV during sleep: overnight heart rate variability is well-measured by optical sensors and is a genuinely useful indicator of recovery and autonomic function
What Trackers Measure Poorly
- Sleep stages: classifying light sleep, deep sleep, and REM from wrist-worn devices has much lower accuracy than device marketing implies. Stage classifications can be wrong 40-50% of the time compared to EEG-based polysomnography.
- Sleep quality as a composite score: proprietary scores are algorithm-specific and not validated against clinical standards. Use them for trend direction, not absolute values.
Orthosomnia: The Tracker Paradox
A recognised phenomenon called orthosomnia describes anxiety about tracker sleep scores leading to worse sleep. If tracking your sleep is increasing rather than reducing anxiety about sleep quality, the data is counterproductive. The tracker serves you, not the reverse.
What Wearables Can and Cannot Tell You in Practice
Use sleep tracking for trends over weeks and months, not single-night scores. Trust duration, timing, and HRV data more than stage classifications. If your scores are consistently poor but your daytime function is fine, trust the function over the score.
