5 things experts want you to know about the data in sleep-tracking devices

Your watch says you had three hours of deep sleep. Should you believe it?

Millions of people rely on phone apps and wearable devices like rings, smartwatches and sensors to monitor how well they’re sleeping, but these trackers don’t necessarily measure sleep directly. Instead, they infer states of slumber from signals like heart rate and movement, raising questions about how reliable the information is and how seriously it should be taken.

The U.S. sleep-tracking devices market generated...