What the science says about their reliability and effectiveness
The ubiquity of smartphones and wearables makes it easy to count steps, spends, and snoozes in the name of optimizing health and happiness. But how reliable is consumer sleep tracking?
The challenge for mainstream sleep trackers is to achieve practical insights into sleep quality without being able to measure sleep directly. A direct scientific assessment of sleep is known as polysomnography, and it measures several important variables in order to profile sleep accurately. These often include recordings of eye movements, muscle activity, breathing, and brain activity. This comprehensive analysis, which is usually done in a sleep lab, requires multiple devices and can be cumbersome and expensive.
Consumer-focused sleep trackers need to take shortcuts. This means that most of them rely on correlations between particular body movements and sleep.
Generally speaking, the body acts differently during different phases of sleep: For example, REM sleep involves less muscle activity and movement than non-REM sleep, and people naturally move more when they are awake than when they are asleep. Therefore, devices that measure body movements, either using a microphone to record the sound of movement or an accelerometer to detect changes in device position, could potentially infer whether a person is sleeping or not.
But how strong is the relationship between body movement and sleep quality? Many people who struggle to fall asleep actually remain relatively still while lying in bed, so how would a device know that they are awake? The simple answer is that the device can’t know for sure, and there will inevitably be errors in sleep data. Any healthy skeptic will want to know how serious those errors are; are they significant enough to warrant throwing out the sleep tracker altogether?