Sleep Tracking Accuracy: A Comparison of Oura Ring Gen 3, Fitbit Sense 2, and Apple Watch Series 8

With consumer interest in sleep tracking on the rise, the accuracy of sleep monitoring devices is an important topic. While many rely on these devices for insight into their sleep patterns, few studies evaluate how well they perform to a scientific level of accuracy. Most people and almost all reviewers either don’t comment on sleep stages or say they feel right or are correct.

A recent study compared three popular previous-generation sleep trackers: the Oura Ring Gen 3, Fitbit Sense 2, and Apple Watch Series 8. These devices were tested against the gold standard of sleep assessment, polysomnography (PSG), to detect sleep stages and duration.

One issue with correctly identifying sleep stages is that PSG requires human scoring, and humans make mistakes. ‘Proper’ PSG will require multiple human scorers to agree, which is expensive and rarely undertaken.

A further issue is that a Polysomnograph with one scorer may only be 80% accurate. So if a wearable is 80% as accurate as a thing that is 80% accurate, potentially  that isn’t very accurate at all.

Study Overview

Thirty-five participants aged 20–50 took part in a single-night in-patient study. Each person wore all three devices, and their sleep was monitored using PSG. The devices were tested on their ability to distinguish between sleep and wakefulness and their accuracy in identifying different sleep stages, including light sleep, deep sleep, and REM.

Sleep vs. Wake Detection

All three devices performed well when detecting whether participants were asleep or awake. The sensitivity (the ability to correctly identify sleep) was 95% or higher across the board for the Oura Ring, Fitbit, and Apple Watch. This means that all three devices were reliable in determining when users were asleep, offering reassurance for those using them to monitor their general sleep patterns.

Accuracy in Identifying Sleep Stages

When it came to identifying specific sleep stages, there were some noticeable differences in performance:

• Oura Ring Gen 3: This device showed solid overall accuracy, with sensitivity ranging from 76.0% to 79.5% and precision (correctly identifying the sleep stage) from 77.0% to 79.5%. Importantly, its estimates for light, deep, and REM sleep were not significantly different from the PSG results, making it the most accurate.
• Fitbit Sense 2: The Fitbit’s sensitivity ranged from 61.7% to 78.0%, with a 72.8% to 73.2% precision. While still relatively accurate, the Fitbit overestimated light sleep by 18 minutes and underestimated deep sleep by 15 minutes on average. Despite these discrepancies, it offered a reasonable level of accuracy for most users.
• Apple Watch Series 8: The Apple Watch had the broadest range of sensitivity, from 50.5% to 86.1%, and precision between 72.7% and 87.8%. While the device accurately detected sleep, it tended to underestimate the time spent awake by 7 minutes and deep sleep by 43 minutes while overestimating light sleep by 45 minutes. While not as precise as the Oura, it still provided helpful data for users wanting a general sense of their sleep patterns.

Key Findings

The Oura Ring Gen 3 emerged as the most accurate device, performing closest to PSG in detecting wakefulness and different sleep stages. Its precise tracking of light, deep, and REM sleep makes it a top choice for those seeking detailed insights into their sleep.

The Fitbit Sense 2 and Apple Watch Series 8 also performed well, though they showed some limitations in identifying specific sleep stages. The Fitbit overestimated light and deep sleep, while the Apple Watch’s estimates for light and deep sleep were further off.

Tech Notes:

Apple has since released two subsequent Watch models, but both probably use the same core sleep sensors and algorithms as the Watch under test.

Oura has recently released Ring Gen 4, which is claimed to be more accurate.

Fitbit Sense 2 is a current-gen Fitbit model, but the brand’s future is uncertain. Fitbit’s owner, Google, has released newer tech under the Pixel brand.

Take Out

Different kinds of PSG themselves have various degrees of accuracy.

All three wearables offer reliable insights for those interested in tracking sleep duration and general patterns. They are accurate in detecting when you’re asleep versus awake. However, if you want more detailed information about your sleep stages, the Oura Ring Gen 3 is the best performer in this study. (Study Head, Dr R. Robbins, is an Oura Ring Medical Advisory Board member and reports consulting fees from Oura Ring Inc.).

Understanding how these devices compare to the gold standard of sleep tracking (PSG) can help consumers make informed choices about the metrics they consume from wearables. While consumer devices are becoming increasingly accurate, it’s important to remember that none fully match the precision of laboratory-based assessments like PSG.