Sleep Trackers May Not Show How Well You Slept

After a poor night's sleep, you may wake feeling anything but refreshed - and your sleep tracker may offer little clue about the reason.

A new review of the evidence suggests wrist-worn sleep trackers, including Fitbit devices and the Apple Watch, may not accurately reflect how well you truly slept.

Although these widely used devices can record the amount of time you spent asleep, they may miss the information that reflects whether you were satisfied with your sleep or felt fatigued or refreshed the following day. This is particularly relevant for people with insomnia.

Pulmonologist Narat Srivali of Duke University and physician-scientist Wisit Cheungpasitporn of the Mayo Clinic reported their findings in the journal Sleep Medicine.

"In healthy individuals tracking general wellness, wearables may provide reasonable feedback regarding sleep duration and regularity," they conclude.

"However, in clinical populations experiencing sleep disturbances or mental health conditions, the poor agreement documented in this review suggests that device measurements may provide misleading information that could delay appropriate clinical intervention."

Sleep trackers and perceived sleep quality

The systematic review examined data from five earlier studies involving roughly 2,000 people. It showed that an individual's own view of their sleep frequently differs from what their sleep tracker reports.

Wearable devices accounted for just 2.5 to 16.2 percent of the variation in subjective sleep quality - the measure currently used when diagnosing insomnia.

Put simply, a person can feel they slept poorly while their sleep tracker indicates otherwise.

Across the reviewed studies, people who gave their sleep poor-quality scores - whether because they found it difficult to fall asleep, woke repeatedly, or felt exhausted the next day - did not always receive poor tracker readings.

Indeed, the devices "failed to capture" this subjective experience, particularly in older adults and people with insomnia.

Why sleep tracker data cannot diagnose insomnia

Several studies have assessed wearable sleep trackers against polysomnography (PSG) scores. These record physiological changes during sleep, including the time spent in light, deep and REM sleep.

Yet a person may show normal sleep duration and efficiency in PSG testing while still waking without feeling refreshed.

For that reason, PSG scores alone are not used to diagnose insomnia. Instead, clinicians generally assess the disorder through a patient's described symptoms and sleep diaries.

Srivali and Cheungpasitporn argue that the connection between sleep trackers and sleep quality as experienced by patients "remains poorly characterized".

"Clinicians increasingly encounter patients who self-monitor sleep with commercial wearables and may present device data," they write.

"Our results suggest that such data, while potentially useful for tracking sleep duration patterns over time, should not be considered an adequate substitute for validated subjective sleep quality assessment tools."

The review authors add that patients who describe poor sleep quality despite their sleep tracker results should receive further clinical assessment, "rather than reassurance based solely on device data".

Fitbit, Apple Watch and gaps in sleep measurements

Overall, the studies found poor-to-moderate agreement between sleep-tracker measurements and subjective evaluations of sleep quality.

One study in the review found that a Fitbit showed acceptable agreement with laboratory sleep measurements for 82 percent of good sleepers, but for only 39 percent of participants with insomnia.

This was largely because Fitbit devices did not identify how often people woke up during the night. Among participants with insomnia, they overstated total sleep time by around half an hour and sleep efficiency by approximately 8 percent.

The devices also aligned less closely with how older adults and people with depression believed they had slept.

This points to a gap between objective and subjective measures of sleep that sleep trackers do not capture.

"An individual with depression may perceive sleep as poor despite objectively normal parameters, while another with fragmented sleep may report satisfaction if expectations are met," the review authors explain.

"The subjective experience is intimately connected to psychological well-being in ways that movement and heart rate patterns cannot capture."

Four of the five studies reviewed used Fitbit devices, and only one examined the Apple Watch. Other sleep trackers may therefore provide more dependable measures of sleep satisfaction, but this has yet to be investigated.

For example, a recent Norwegian survey of 461 people who use or had used a sleep tracker found that few said their sleep quality had improved. Participants with insomnia were also more likely to experience negative thoughts and concerns about sleep.

The review's findings indicate that someone concerned about a sleep disorder such as insomnia may give their doctor more useful information through a sleep diary than through sleep-tracker data.

For people with depression, meanwhile, sleep trackers may point to symptoms such as 'sleeping too much'. Beyond this, however, the review authors say that "they fail to detect most other depression symptoms that self-reported sleep quality successfully identifies."

At present, sleep tracker users should recognise that these devices may be of limited use for assessing their satisfaction with sleep. They should not become anxious about their results.

The study was published in Sleep Medicine.

This article was fact-checked by Clare Watson and edited by Clare Watson. Although we take pride in our process, we are only human. If you notice an error, please let us know.