Understanding Watch-Based Rem Sleep Detection

Sleep tracking watches are a popular way to monitor sleep patterns and cycles. They are often worn on the wrist and work by monitoring body movements and, in some cases, heart rate changes during sleep. The data collected by these watches can help users understand their sleep quality and duration, as well as identify any issues that may be causing sleep disturbances. While these devices can be useful for those who want to track their sleep, it's important to note that they may not always provide completely accurate readings and can sometimes cause more anxiety about sleep quality.

Heart rate and movement detection

Sleep tracking devices are usually in the form of a watch worn on the wrist. They can be used to monitor sleep duration, quality, phases, environmental factors, and lifestyle factors.

Sleep tracking watches use actigraphy, or wrist movement detection, to track sleep patterns. They primarily use a gyroscope or accelerometer to track movement and detect the stage of the sleep cycle. Watches that incorporate heart rate data tend to be slightly more accurate when measuring sleep duration because heart rate fluctuates during different sleep stages.

Heart rate tends to slow as the body moves into deeper sleep. During REM sleep, the body is paralysed, which also affects heart rate. Sleep tracking watches use these changes in heart rate to estimate the stage of sleep.

However, research has found that sleep trackers are only accurate 78% of the time when identifying sleep versus wakefulness. This accuracy drops to around 38% when estimating how long it took the wearer to fall asleep.

Sleep tracking watches can be useful for helping users recognize patterns in their sleep habits. However, they are not always accurate, and in some cases, they may cause more harm than good. For example, they could cause anxiety in people who experience poor sleep quality or have mental health conditions.

Actigraphy

Actigraphs are typically worn on the wrist of the non-dominant hand, but can also be worn on the ankle, or as a clip-on device attached to clothing. They are lightweight, and often incorporated into watches or other wearable devices. They are useful for helping to determine whether disruptions in the sleep-wake cycle exist, as may occur in many different sleep disorders.

Sleep quality

Sleep trackers use actigraphy, or movement detection, to track sleep patterns. They use a gyroscope or accelerometer to track your movements and detect which stage of the sleep cycle you're in. They can also use heart rate sensors to monitor your heart rate, which lowers as you move into deeper sleep and varies depending on the sleep stage.

Some sleep trackers allow you to input lifestyle factors, such as caffeine consumption, meal times, and stress levels, which can affect sleep quality. Environmental factors, such as light and temperature, can also impact sleep quality and are recorded by some devices.

While sleep trackers can provide insights into your sleep quality, they may not always be accurate. They often measure inactivity as a surrogate for estimating sleep and make guesstimates about your sleep stages. For more accurate data, a medical sleep study that monitors brain waves is necessary.

Sleep duration

Sleep tracking watches are a popular way to monitor sleep duration. These devices can record when you fall asleep and when you wake up in the morning by tracking the time you're inactive. They can also detect interrupted sleep, letting you know if you're tossing and turning or waking up during the night.

Sleep tracking watches work by monitoring your body movements and, sometimes, your heart rate as you sleep. They use an algorithm to estimate how much time you spent asleep based on this data. For example, as you move into deep sleep, your heart rate slows, and the body is paralysed during REM sleep. This data helps the watch track and monitor your sleep cycle closely.

However, it's important to note that sleep tracking watches are not always accurate. Research has shown that compared to polysomnography tests – which experts use to diagnose sleep disorders – sleep trackers are only accurate 78% of the time when identifying sleep versus wakefulness. This accuracy drops to around 38% when estimating how long it took participants to fall asleep. This is because movement provides few clues about what stage of sleep someone is in, and many sleep devices struggle to differentiate one stage of sleep from another based on motion alone.

Despite their limitations, sleep tracking watches can be a useful tool for helping you recognize patterns in your sleep habits. For example, you might find that you feel more energetic when you sleep from 11 p.m. to 7 a.m. instead of 10 p.m. to 6 a.m. or that your sleep is disrupted if you have caffeine after lunchtime.

Sleep phases

Sleep is divided into two broad categories: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. The former is characterised by rapid eye movements and is associated with dreaming, while the latter is further divided into three stages, N1 to N3, which become progressively deeper.

The first stage of NREM sleep, N1, is the lightest stage and usually lasts just a few minutes. During this stage, the body and brain activities start to slow, with periods of brief movements. It is easy to wake someone up during this stage, but if they are not disturbed, they will quickly move into N2 sleep.

The second stage, N2, is when the body enters a more subdued state, with a drop in temperature, relaxed muscles, and slower breathing and heart rate. Brain waves show a new pattern, and eye movement stops. N2 sleep can last for 10 to 25 minutes during the first sleep cycle, and each successive stage can become longer, with people typically spending about half their sleep time in this stage.

The third stage, N3, is the deepest stage of sleep, and it is harder to wake someone up during this phase. Muscle tone, pulse, and breathing rate decrease as the body relaxes even further. This stage is critical for restorative sleep, allowing for bodily recovery, growth, and a boost to the immune system. It also contributes to insightful thinking, creativity, and memory.

After N3 sleep, the body moves back into N2 sleep, which is the gatekeeper of REM sleep. During REM sleep, brain activity picks up, nearing levels similar to when a person is awake. The body experiences atonia, a temporary paralysis of the muscles, except for the eyes and the muscles that control breathing. Dreams occur during this stage, which is known for being the most vivid.

Throughout the night, a person will typically go through four to six sleep cycles, each lasting about 90 to 110 minutes. The first sleep cycle is often the shortest, while later cycles tend to be longer. As the night goes on, REM stages get longer, and more time is spent in REM sleep instead of N3 sleep.

Frequently asked questions