Does the sleep tracking feature on my smartwatch have any real value?

Smartwatches measure sleep reasonably well compared with smartphone apps alone, but they do not reach the accuracy of a clinical sleep study (polysomnography). What sets devices apart is the algorithm: some are well validated, while others can produce essentially random results. Standardised criteria for comparing devices with one another do not yet exist, so as a consumer it is difficult to know which category your watch falls into.

Extra caution is warranted when it comes to sleep apnoea. Some smartwatches claim to be able to detect it, but reliability varies considerably from device to device. Clinical guidelines require a minimum of six measured signals for a reliable home test. Most smartwatches measure fewer than that. The fact that a device 'detects sleep apnoea' therefore does not mean that result is correct: with a common condition, a random diagnosis can simply be correct by chance.

More interesting is the use of sleep data as part of a broader picture. Six of the nine studies in a systematic review showed that combined data on sleep, steps and heart rate could predict depressive episodes with an accuracy of 81 to 91 percent. Those results are promising, but the studies were small and rarely lasted longer than a year. This does not yet work as an individual tool you can apply at home right now, but it does show that sleep data has the potential to be more than an interesting trivia graph.

In cancer patients, wearables also proved useful for more objective symptom recording and better medication adherence, compared with purely self-reported data. Those are real benefits, although here too the associations are correlational rather than proven causes.