How Does WHOOP Measure Sleep, and How Accurate is It?

So How Accurate is WHOOP?

After over two years of research and validation using gold-standard sleep tracking polysomnography (PSG) data from Central Queensland University and the University of Arizona, WHOOP is even better at measuring sleep stages (awake, light, REM, and deep sleep) making your sleep data more reflective of how your body rests and recovers. Here’s what you can expect:

• More accurate sleep staging: Classification accuracy across the 4 sleep stages has improved by over 7% according to WHOOP internal testing, leading to more precise insights into how much time you spend in each sleep stage. 
• Improved sleep/wake classification: The ability to detect wake from sleep has improved by over 3% via WHOOP internal testing. This means your sleep data more accurately reflects your rest, specifically accounting for short wake periods—like when you move at night.

There have also been additional third party studies of WHOOP having paramount accuracy:

• This Central Queensland University study found that WHOOP is 99.7% accurate in measuring heart rate and 99% accurate in measuring heart rate variability during sleep—levels of accuracy that surpassed all other wearables in the study.
• The same CQU study also found that WHOOP was excellent in identifying sleep when compared to the gold-standard polysomnography (PSG) and outperformed the other devices in calculating total time spent asleep.
• Another study of leading wrist-worn wearables revealed that WHOOP was the most accurate across several categories when compared to the gold-standard PSG. 

This level of precision means you can trust WHOOP to provide sleep insights that help you sleep better—whether that’s adjusting your bedtime, improving sleep efficiency, and beyond.

WHOOP is Built for Sleep

WHOOP was designed from the ground up to provide the most accurate possible sleep tracking. We collect hundreds of data points per second from our 3-axis accelerometer, 3-axis gyroscope, and PPG sensor.

PPG, or photoplethysmography, is a technique that involves measuring blood flow by assessing subtle changes in blood volume in capillaries close to your skin. 

In a PSG sleep study, subjects undergo simultaneous electrocardiogram (EKG), electrooculogram (EOG), electroencephalogram (EEG), and electromyogram (EMG) recordings. Trained technicians then interpret these results, sorting 30-second intervals of data—called an epoch—into one of four sleep stages: Wake, Light, REM, and Slow Wave. While PSG is the gold standard and the most accurate known way to determine sleep stages, it is also expensive, cumbersome, and intrusive. WHOOP used this PSG and WHOOP data to train machine learning models to accurately detect the different sleep stages.

If you’ve ever wondered what the tiny green lights are at the bottom of your WHOOP, they are a very important part of PPG. Between the two green lights, there's a small photo-receptor which measures light. When you shine specific colors (wavelengths) of light onto the skin, blood volume can be measured by looking at the light reflected back from your skin since blood absorbs specific colors and reflects others. Once blood flow is measured, we can then derive heart rate, heart rate variability, and respiratory rate, all of which are used in our sleep staging algorithms.

Why Sleep Is So Important

Tracking and analyzing sleep is central to overall health. Despite its importance, getting healthy sleep is harder than ever before. Gallup poll data shows that modern Americans average more than an hour less sleep per night than 70 years ago. The current epidemic of insufficient sleep increases our risk of cardiovascular disease, cancer, obesity, and even shortens our life expectancy.