Wrist based activity trackers are just beginning to differentiate between the major types of sleep REM, Light and Deep. We used to have to wear EEGs on our heads or unusual monitoring devices strapped to the bed. Pillow (v3.1) is an app for iOS and presently relies on Apple Watch OS 3.0 for movement and heart rate data collection. Sleep start times need to be manually triggered and the app requires a one-time purchase to unlock all the premium features.