Apple has explained some of the methodology behind the way it calculated that 18-hour battery life claim that comes alongside the Apple Watch, suggesting you'll make it for longer with a usable lump on your wrist if you calm down on the impulsive checking of things.
According to Apple's General Battery Information page, the Watch should be good for an aspirational day that unfolds like this and includes:
90 time checks, 90 notifications, 45 minutes of app use, and a 30-minute workout with music playback from Apple Watch via Bluetooth
That's your 18 hours, as long as you started off with a full overnight charge and didn't do anything stupid like wear it to bed or get up at 3:45am.
Other stats tell us it'll take 2.5 hours to fully charge from broken, can manage 7 hours of non-stop activity tracking, and last for 72 hours on super-low-power mode if you restrain yourself and only use it to check the time four times an hour. [Apple via Engadget]