{
 "id": 39605,
 "slug": "whoop-sleep-tracking-what-it-actually-measures",
 "url": "https://agents.willem.com/en/snippets/whoop-sleep-tracking-what-it-actually-measures/",
 "language": "en",
 "title": "Whoop Sleep Tracking — What It Actually Measures",
 "subtitle": "Sleep stages, HRV, respiratory rate — and the limits of wrist-based measurement.",
 "summary": "The Whoop Strap tracks your sleep automatically — stages, duration, disturbances, HRV, and respiratory rate. But how accurate is it really? After months of nightly data, here's what Willem found.",
 "topics": [
  "health",
  "heart",
  "sensors"
 ],
 "created": "2026-04-03T19:41:02.715000+02:00",
 "updated": "2026-04-03T20:17:17.646000+02:00",
 "full_article_url": "https://willem.com/en/2019-12-31_whoop-strap-review-24-7-wearable-sensor/",
 "author": {
  "name": "Willem L. Middelkoop",
  "url": "https://willem.com"
 },
 "note": "A short distillation of a longer willem.com post. Not currently published on willem.com itself.",
 "answers_question": "What does Whoop actually measure while you sleep?",
 "text": "The Whoop measures sleep by tracking heart rate variability (HRV), resting heart rate, and wrist movement throughout the night. It classifies sleep into stages: wake, light, REM, and deep (slow-wave) sleep.\nThe recovery score — Whoop's headline metric — is calculated primarily from your sleep quality, HRV, and resting heart rate. A green recovery means your body is ready for strain; a red one means you need more rest.\nWhat it does well: detecting when you actually fell asleep (not when you got into bed), measuring total sleep time accurately, and tracking trends over weeks and months.\nWhat it struggles with: sleep stage classification is approximate compared to polysomnography. Light vs. deep sleep percentages should be taken as trends, not absolutes.\nFrom Willem's multi-year Whoop review.",
 "markdown_url": "https://agents.willem.com/en/snippets/whoop-sleep-tracking-what-it-actually-measures/snippet.md"
}