Sleep Trackers: What Your Wearable Gets Right, and What It Misses

The market for consumer sleep trackers has exploded, Oura Ring, Apple Watch, Garmin, Whoop, Withings. Each promises to decode your nightly recovery. And they've genuinely improved. But a fundamental gap remains between what wearables measure and what a clinical polysomnography (PSG) sleep study measures.

What They Measure Well

• ✓Total sleep duration, fairly accurate within 20-30 minutes for most people
• ✓Heart rate variability (HRV), reliably captured for trend analysis via optical sensor
• ✓Resting heart rate, well-validated across most platforms
• ✓Sleep regularity, consistency of bedtime and wake time accurately tracked
• ✓Week-over-week trends, where these devices genuinely shine

Where They Fall Short

Sleep stage classification is where consumer devices diverge most from clinical truth. PSG uses EEG (brainwaves), EOG (eye movements), and EMG (muscle activity) to classify stages. Wearables infer stages from movement, heart rate patterns, and sometimes skin temperature, achieving 65-85% accuracy against PSG, depending on device and stage. Deep sleep detection is particularly unreliable.

The Orthosomnia Problem

Sleep researchers have identified "orthosomnia", anxiety about tracker data that itself impairs sleep quality. People lie awake worrying about their sleep scores. The right mindset: treat wearable sleep data as directional signal over weeks, not nightly truth.