Whoop's Rem Sleep Detection: Understanding The Science And Technology

The WHOOP strap is a wearable device that estimates sleep based on measures of movement and heart rate. It uses a 3-axis accelerometer and reflectance photoplethysmography to measure heart rate, heart rate variability, ambient temperature, motion and movement, and skin response.

The device's sleep detection and staging algorithms are trained using data from in-laboratory polysomnography (PSG) testing, which involves simultaneous electrocardiogram (EKG), electrooculogram (EOG), electroencephalogram (EEG), and electromyogram (EMG) recordings. PSG is considered the gold standard for measuring sleep, but it is expensive, cumbersome, and intrusive.

The WHOOP sleep staging algorithm has been shown to have high levels of agreement with PSG testing, with a precision error of 1 beat per minute for heart rate and 1 breath per minute for respiratory rate. It accurately detects sleep duration with a precision of 17.8 minutes and has highly accurate detection of REM and Slow Wave (deep) sleep.

How does WHOOP measure sleep?

WHOOP is a wearable device that measures sleep based on movement and heart rate. It uses actigraphy and photoplethysmography to estimate sleep. Actigraphy is a technique that measures sleep by assessing superficial changes in blood volume. Photoplethysmography (PPG) involves shining specific colours of light onto the skin, as blood absorbs and reflects different colours. This allows the device to measure blood flow, and subsequently, heart rate, heart rate variability, and respiratory rate.

The WHOOP device collects hundreds of data points per second using its accelerometer, gyroscope, and PPG heart rate sensor. It also measures capacitive touch and temperature, although these are not used in the sleep algorithm.

The sleep algorithm turns the data from the accelerometer, heart rate, heart rate variability, and respiratory rate into the sleep analysis that users receive. To achieve this, WHOOP partnered with a sleep centre and had hundreds of subjects undergo in-laboratory polysomnography (PSG) testing while wearing WHOOP. PSG is the gold standard for determining sleep stages and involves simultaneous electrocardiogram (EKG), electrooculogram (EOG), electroencephalogram (EEG), and electromyogram (EMG) recordings. Trained technicians then manually interpret the results, sorting each 30-second chunk of data into one of four sleep stages: Wake, Light, REM, and Slow Wave.

WHOOP's sleep staging algorithm brings together many physiological variables, or "features", to determine sleep stages. Some of these are more pronounced than others. For example, respiratory rate is fairly constant during slow-wave sleep, while during REM sleep it is increased and more variable.

WHOOP's sleep tracking has been validated in multiple studies. One study found that WHOOP accurately detects sleep duration with a precision of 17.8 minutes, with highly accurate detection of REM and Slow Wave sleep. Another study found that WHOOP was the most accurate when compared to the gold-standard PSG.

How accurate is WHOOP in measuring sleep?

WHOOP is a wearable device that estimates sleep based on measures of movement and heart rate derived from actigraphy and photoplethysmography. It has been validated against polysomnography (PSG), the gold standard of sleep tracking, in multiple studies.

Accuracy of WHOOP in Measuring Sleep

WHOOP has been shown to have high sensitivity in detecting sleep and good overall agreement with PSG. It has been found to have high levels of agreement with PSG in detecting sleep and wake states, with sensitivity and specificity to sleep reported at 89% and 95% respectively. WHOOP has also been shown to have high accuracy in detecting REM and Slow Wave (deep) sleep.

However, WHOOP tends to overestimate total sleep time and underestimate wake duration. It also has lower specificity in detecting wake states, ranging from 18% to 80% across studies. This is because it is an indirect measure of sleep that does not record brain activity.

WHOOP collects hundreds of data points per second using its accelerometer, gyroscope, and PPG heart rate sensor. It measures capacitive touch and temperature but does not use data from these sensors in its sleep algorithm. PPG, or photoplethysmography, is a technique that involves measuring blood flow by assessing superficial changes in blood volume. WHOOP shines specific colours (wavelengths) of light onto the skin to measure blood volume by looking at the light reflected back. Once blood flow is measured, WHOOP can then derive heart rate, heart rate variability, and respiratory rate, which are used in its sleep detection and staging algorithms.

What are the four main sleep stages?

Sleep is one of the most important activities for good health. During sleep, your body cycles through four different stages, all of which affect your brain waves, muscle relaxation, recovery, and breathing.

The first three stages are referred to as "non-REM sleep", and the fourth is "REM sleep". Here is a breakdown of each stage:

Stage 1

This is the typical transition from wakefulness to sleep and generally lasts only a few minutes. Stage 1 is the lightest stage of sleep. If you wake someone up during this stage, they might report that they were not actually asleep.

During this stage:

• Eye movements are typically slow and rolling.
• Heartbeat and breathing slow down.
• Muscles begin to relax.
• You produce low-amplitude mixed-frequency waves in the theta range (4 to 7 Hz).

Stage 2

Stage 2 sleep comprises the largest percentage of total sleep time and is considered a lighter stage of sleep from which you can be awakened easily. This is the stage before you enter deep sleep.

During this stage:

• Heartbeat and breathing slow down further.
• Body temperature drops.
• Eye movements stop.
• Breathing and heart rate become more regular.
• The brain also begins to produce bursts of rapid, rhythmic brain wave activity, known as sleep spindles.

Stage 3

Stage 3 is the deepest sleep stage. It is known as slow-wave, or delta, sleep. Your body performs a variety of important health-promoting functions in this final non-REM stage.

During this stage:

• It is difficult to wake someone up from sleep.
• Heartbeat and breathing slow down further.
• Muscles are completely relaxed.
• Blood pressure drops.
• Delta brain waves, which are slower brain waves that indicate healing and restorative sleep, are present.

Stage 4: REM Sleep

REM sleep typically happens 60 to 90 minutes after falling asleep. This is when you tend to have vivid dreams.

During this stage:

• Eye movements become rapid.
• Breathing and heart rate increase and become more variable.
• Muscles become paralysed, but twitches may occur.
• Brain activity is markedly increased.

Sleep Cycles

When you fall asleep, you cycle through all of these stages of sleep multiple times, roughly every 90 minutes or so. A full sleep cycle is generally around 90 minutes long, and four to five cycles occur during a typical night of sleep.

What is sleep latency?

Sleep latency, or sleep onset latency, is the amount of time it takes to fall asleep once you go to bed. It is an important indicator of the overall quality of your sleep. Ideally, sleep latency should be between 10 and 20 minutes. If it takes longer than that, it can affect your sleep efficiency, or the percentage of time spent asleep while in bed. If you nod off as soon as your head hits the pillow, it could be a sign that you're not getting a healthy amount of sleep each night.

Long sleep latency, or taking more than 20 minutes to fall asleep, can be caused by a variety of factors such as taking frequent naps and experiencing chronic pain. It can also be a sign of a more serious problem, such as insomnia. Short sleep latency, or falling asleep almost immediately, is often a sign of sleep debt, which is the overall effect of not getting enough sleep. Conditions such as narcolepsy and idiopathic hypersomnia can also lead to short sleep latency.

Healthcare providers may recommend a test to measure sleep latency if they suspect a sleep disorder. There are three common ways to do this: a polysomnogram, multiple sleep latency test, and maintenance of wakefulness test. A polysomnogram is an overnight sleep study done in a sleep laboratory or at home, which involves sleeping with sensors attached to the body to measure heart rate, breathing, and brain waves. The multiple sleep latency test evaluates how quickly you fall asleep for a daytime nap after a night of sleep and is used to diagnose narcolepsy and idiopathic hypersomnia. The maintenance of wakefulness test, on the other hand, assesses how long you can stay awake for a certain period.

The WHOOP strap is a wearable device that has been validated as a reasonable method for estimating sleep, particularly for 2-stage categorisation (wake and sleep). It uses measures of movement and heart rate derived from actigraphy and photoplethysmography, respectively, to estimate sleep. WHOOP has been shown to have excellent agreement with polysomnography, the gold standard of sleep tracking, in detecting sleep duration and wakefulness. It also accurately measures heart rate and respiratory rate, which are essential for accurate sleep staging.

How does WHOOP help with sleep consistency?

Sleep consistency is going to bed and waking up at the same time, in order to preserve a consistent circadian rhythm. The WHOOP app provides a Sleep Page that offers a summary of the previous night's sleep, including a Sleep Performance score (the hours of sleep achieved compared to the amount of sleep needed) and statistics such as Time in Bed, Disturbances, and Latency. WHOOP's Sleep Performance is reported on a 0:100% scale, with 100% indicating that the user achieved all the sleep they needed for the night.

WHOOP uses advanced sleep monitoring technology to accurately capture the true amount of time a user is sleeping, and not just the amount of time spent in bed. The device collects hundreds of data points per second using its accelerometer, gyroscope, and PPG heart-rate sensor. WHOOP heart rate during sleep was shown to have excellent agreement with EKG, the gold standard, averaging a precision error of 1 beat per minute across 32 participants.

The WHOOP Sleep Page also provides detailed statistics, including:

• Disturbances—the number of times a user wakes up throughout the night, which are typically not conscious awake times but rather reflect times when the user shifted in their sleep or came briefly out of a light phase of sleep.
• Efficiency—how long a user was asleep relative to how long they were in bed.
• Respiratory Rate—the average number of breaths achieved per minute throughout the user's sleep.
• Sleep Stages—shows the Sleep Staging for the selected night, including Slow Wave Sleep (SWS), REM sleep, Light Sleep, and Awake.

WHOOP members average slightly more than 7 hours of sleep each night, with women averaging 7:20 and men getting 7:03 hours of sleep. However, the amount of sleep WHOOP members get and how much they need are not the same thing. Sleep needs can change daily, based on factors including how much sleep the user got the night before and the amount of exercise and stress accumulated throughout the day.

Frequently asked questions