Smartwatches often misjudge people’s sleep quality

A review of research published by ERR found that smartwatches such as Fitbit and Apple Watch may estimate how long a person sleeps reasonably well, but their readings often do not match how people assess their own sleep quality.

US researchers analysed five earlier studies involving nearly 2,000 people, comparing wearable-device readings with laboratory sleep studies and participants’ own feedback. The data showed that wearables explained no more than 16 percent of a person’s subjective sleep experience.

For people who slept well, Fitbit readings matched laboratory measurements in 82 percent of cases. Among people with insomnia, the figure was only 39 percent.

The devices overestimated the total sleep time of people with insomnia by an average of 30 minutes and rated their sleep efficiency eight percentage points higher than it actually was.

The researchers attributed the discrepancy to the devices’ technical limitations. Wrist monitors rely mainly on movement and heart rate, which cannot detect brief nighttime awakenings with sufficient accuracy.

They also do not account for a person’s emotional state. For example, someone with depression may perceive their sleep as poor even when the device records normal physiological indicators.

For this reason, the researchers said that people who suspect they have a sleep disorder may find it more useful to keep a traditional sleep diary when consulting a doctor.

The findings were published in the journal Sleep Medicine.