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All-day battery life is based on 18 hours with the following use: 90 time checks, 90 notifications, 45 minutes of app use, and a 30-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours.
The official Apple Watch battery life specs are listed as both all day and up to 18 hours, according to Apple. Note: there are still 24 hours in a day. Yes, that all day claim technically syncs up with a normal sleep schedule and it takes only two and half hours to fully recharge.