What does a sleep tracker actually measure when it tells you that you slept badly—and how much should you trust the score?

Stanford sleep and circadian researcher Dr. Jamie Zeitzer joins Neil Hedley to unpack the gap between a precise-looking number and the evidence behind it. They discuss why wrist-based devices can estimate sleep duration reasonably well for some people yet struggle when sleep is fragmented, what proprietary sleep scores leave unexplained, and why a single night's rating may be less useful than patterns over time.

Neil shares how wearable data prompted him to seek a formal sleep assessment. They also explore what a more useful future could look like: changes that prompt better questions for a clinician, links between daytime behavior and nighttime sleep, and feedback about a bedroom environment that could be adjusted.

Important distinction: Consumer sleep trackers estimate aspects of sleep; a sleep score is not a medical diagnosis. The future applications discussed are possibilities, not established benefits of every device.

Further reading: Jamie M. Zeitzer, The utility of sleep wearables—well, that depends, SLEEP (2024), read the editorial. Zeitzer, A time for change: actigraphy in the age of consumer wearables, SLEEP (2026), read the editorial. Dr. Zeitzer's Stanford profile.