Whoop tracks your heart rate, heart rate variability, sleep stages, skin temperature, respiratory rate, blood oxygen levels, and physical strain throughout the day and night. Unlike a smartwatch, it has no screen. Everything flows to the app, where the data is distilled into three core scores: Recovery, Strain, and Sleep. Here’s what each metric actually measures and why it matters.
The Sensors Behind the Data
The Whoop 4.0 band contains five LEDs (three green, one red, one infrared) and four photodiodes pressed against your skin. These light sensors read your pulse by detecting tiny changes in blood flow, a method called photoplethysmography. It’s the same basic technology in an Apple Watch or Fitbit, but Whoop runs it continuously, even during sleep, rather than sampling intermittently.
An accelerometer and gyroscope track motion, which feeds into step counts, exercise detection, and muscular load estimates. A dedicated skin temperature sensor rounds out the hardware, giving the band a way to flag potential illness or hormonal shifts.
Recovery Score
Every morning, Whoop gives you a Recovery score from 0 to 100%, telling you how ready your body is for stress. Four physiological inputs determine that number:
- Heart rate variability (HRV): the variation in time between heartbeats, measured overnight while you sleep. Higher HRV generally signals a well-recovered nervous system. Whoop calculates this from your slowest, most stable period of sleep to avoid noise from tossing and turning.
- Resting heart rate (RHR): a lower overnight RHR means your heart is pumping efficiently. When your RHR creeps up compared to your personal baseline, it often signals incomplete recovery, dehydration, or stress.
- Sleep performance: the percentage of sleep you actually got versus how much Whoop calculated you needed. Getting six hours when your body needed eight will drag your Recovery down.
- Respiratory rate: breaths per minute during sleep. This metric is remarkably stable night to night for most people, so even a small sustained increase can flag that something is off, like an oncoming illness.
The score is color-coded: green (67–100%) means you’re primed for a hard effort, yellow (34–66%) suggests moderate activity, and red (0–33%) is a signal to take it easy.
Strain Score
Strain measures how much total stress you put on your body during the day, scored on a 0 to 21 scale. The scale is logarithmic, which means going from 10 to 15 requires significantly more effort than going from 5 to 10. Whoop tracks strain both for your full day and for individual workouts.
Two components feed the score. Cardiovascular load tracks how high your heart rate gets and how long it stays elevated. Muscular load captures the physical work your muscles perform, accounting for both volume and intensity. A full-body squat generates more muscular load than a bench press because more of your body mass is moving. Movement speed and how close you push toward failure also factor in.
For strength-based activities like weightlifting, HIIT, and functional fitness, Whoop automatically estimates muscular strain based on the activity type and duration, using data derived from millions of logged sessions. You can get a more precise number by using the Strength Trainer feature in the app, where you log sets, reps, and weights in real time. The band’s accelerometer and gyroscope measure each rep’s speed and intensity, giving you a detailed picture of the work your muscles did. Activities like yoga, Pilates, and barre have their muscular load calculated automatically without any manual logging.
Sleep Tracking
Whoop breaks your sleep into stages: light sleep, slow-wave (deep) sleep, REM sleep, and time awake. It detects when you fall asleep and wake up automatically, so you don’t need to press a button or set an alarm in the app.
A validation study comparing Whoop to polysomnography (the gold-standard clinical sleep test) found 89% agreement when simply distinguishing sleep from wakefulness. When classifying all four sleep stages individually, agreement dropped to 64%. The band was best at identifying REM sleep (70% sensitivity) and deep sleep (68%), while detecting wakefulness was the weakest point at 51%. In practical terms, your total sleep time will be fairly accurate, but the exact breakdown of stages should be treated as an estimate rather than a clinical measurement.
Based on your sleep data and accumulated strain, Whoop calculates a sleep need each night. Sleep performance is the ratio of actual sleep to that target, expressed as a percentage.
Skin Temperature
Rather than giving you a raw temperature reading, Whoop shows how your overnight skin temperature deviates from your personal baseline. That baseline is built from 90 nights of data, so it’s highly individualized. Deviations can indicate the onset of illness, changes in your sleep environment (like a warmer room), or shifts in the menstrual cycle. The trend over days is more useful than any single night’s reading.
Respiratory Rate
Whoop measures your breathing rate during sleep and reports it as breaths per minute. Because this number barely changes from night to night for most people, a noticeable spike can be one of the earliest signs your body is fighting something. The app’s Health Monitor feature lets you see your daily respiratory rate, resting heart rate, and HRV side by side so you can spot patterns. A large, sudden, and sustained increase outside of exercise is the signal worth paying attention to.
Blood Oxygen (SpO2)
Using its red and infrared LEDs, Whoop estimates blood oxygen saturation overnight. This reading reflects how effectively your lungs are delivering oxygen to your bloodstream. Consistently low readings could point to breathing issues during sleep, including sleep apnea, though Whoop is not a medical device and shouldn’t replace a clinical evaluation.
Activity and Workout Detection
Whoop can automatically detect certain activities based on your heart rate and movement patterns, or you can start a workout manually in the app. The platform supports over 80 activity types, from running and cycling to CrossFit, swimming, and golf. Each activity gets its own Strain score so you can compare the cardiovascular and muscular cost of different workouts over time.
The Strength Trainer feature goes deeper for resistance training. It tracks individual exercises, estimates which muscle groups were involved, and assigns muscular load based on effective mass, movement speed, and proximity to failure. Recent updates improved motion detection for unilateral exercises like single-arm rows or lunges, so each side of the body is tracked more precisely.
Daily Journal and Behavioral Tracking
Beyond the automatic sensor data, Whoop includes a daily journal where you can log behaviors like alcohol consumption, caffeine intake, screen time before bed, meditation, and supplements. Over time, the app correlates these entries with your Recovery and Sleep scores, showing you which habits actually move the needle for your body. This is one of the features that separates Whoop from simpler trackers: it doesn’t just collect data, it tries to connect your choices to your outcomes.