Have the sleep timer reset automatically when the phone is being used.
When I have the sleep timer turned on, it would be great if the timer would reset when I'm using my phone /when the screen is on.
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johnm commented
Smart audiobook reader does something like this and it is very handy. It resets the sleep timer if it detects a "shake". It means that I can set a short sleep timer but that playback doesn't get paused if I'm walking around with or using the phone.
I would also suggest reseting the countdown if the user interacts with the player (pressing pause/play/ff/rewind/swipe). It is a little annoying to just press play and then have the countdown time expire shortly after.