Apple iPhone OS 3.1 - Manual (Page 134)

Timer
Set the timer: Tap Timer, then flick to set the number of hours and minutes. Tap Start
to start the timer.
Choose the sound: Tap When Timer Ends.
Set a sleep timer: Set the timer, then tap When Timer Ends and choose Sleep iPod.
When you set a sleep timer, iPhone stops playing music or video when the timer ends.
If you start the timer and then switch to another iPhone application, the timer
continues running.
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