In recent years, smartwatches and fitness trackers have become increasingly popular tools for monitoring sleep. Many devices promise to analyze sleep duration, quality, and even the different sleep stages, providing users with a daily “sleep score” that summarizes how well they rested.

But how reliable are these measurements? And what exactly do these devices measure during the night?

How smartwatches measure sleep

Most smartwatches estimate sleep using a combination of sensors that monitor physiological signals while you sleep. The main ones include:

• accelerometers, which detect body movements
• heart rate sensors, which track changes in heart rate during the night
• heart rate variability (HRV), associated with different physiological states during sleep

Using proprietary algorithms, these signals are analyzed to estimate when a person falls asleep, how long they sleep, and how much time is spent in different sleep stages.

However, these devices do not directly measure brain activity. Instead, sleep stages are inferred from indirect signals, meaning the results are estimates rather than direct measurements.

Estimated data vs. clinical measurements

In sleep medicine, the gold standard for measuring sleep is polysomnography, a clinical test that simultaneously records brain activity, breathing, eye movements, and muscle tone.

Research published in the journal Sleep suggests that consumer wearables can be reasonably accurate in estimating total sleep time, but they are less precise when identifying specific sleep stages such as light sleep, deep sleep, or REM sleep.

Several studies have found that wearables can detect whether someone is asleep or awake during the night with accuracy rates between 80% and 90%. However, their ability to correctly classify sleep stages is significantly lower.

A review published in npj Digital Medicine also found that many consumer devices tend to overestimate sleep duration, particularly when a person remains still in bed while actually being awake.

When sleep tracking can be useful

Despite these limitations, sleep-tracking devices can still provide valuable insights.

Rather than focusing on the accuracy of a single night, wearable devices are particularly useful for identifying sleep trends over time. By tracking sleep patterns over weeks or months, users may notice consistent changes such as:

• later bedtimes
• reduced total sleep duration
• the impact of travel, stress, or exercise on sleep patterns

According to research published in the Journal of Clinical Sleep Medicine, wearable sleep trackers can help increase awareness of sleep habits, which may encourage healthier behaviors and more consistent routines.

How to interpret sleep data correctly

To use these devices effectively, it is important to treat their data as guidelines rather than medical diagnoses.

Smartwatches can provide useful information about sleep behavior, but they cannot replace clinical evaluation for sleep disorders such as chronic insomnia, sleep apnea, or excessive daytime sleepiness.

Instead of focusing too closely on a nightly sleep score, it is more helpful to observe longer-term patterns and combine those insights with healthy habits and a supportive sleep environment.

Because even in the era of wearable technology, truly restorative sleep still depends on a balance of daily routines, sleep environment, and overall wellbeing.