The Best Sleep Tracker
for Apple Watch

iPhone-only sleep trackers rely on a single accelerometer placed on your bedside table — a blunt instrument that picks up partner movement, pets, and ambient vibration. Apple Watch is strapped to your wrist, directly over your radial artery, giving Snollo three independent data streams that iPhone cannot access: a continuous optical heart rate sensor, a pulse oximeter for blood oxygen saturation, and a six-axis accelerometer that registers the micro-movements of genuine sleep stages.

This matters because REM sleep, deep sleep, and core sleep (what other apps call Light) each have distinct physiological signatures. Heart rate dips during deep sleep. REM is marked by muscle atonia — near-zero movement — combined with elevated, irregular heart rate. Core sleep shows characteristic spindle oscillations in movement data. Without wrist-worn sensors, these differences are invisible.

Apple Watch Series 4 and later sample heart rate every second during sleep, rather than the five-minute polling used for daytime activity. Combined with SpO2 readings and fine-grained accelerometer data, this gives Snollo's on-device models enough signal density to distinguish sleep stages well — useful for trends and night-over-night comparison.

The result: you wake up to a precise breakdown of your night — how long you spent in each stage, when your sleep efficiency dipped, and whether your blood oxygen stayed in a healthy range. All of this is computed on your iPhone while you sleep, never uploaded to any server.