On the iPhone:
Do you enjoy listening to music while falling asleep. The problem with that is it'll still be playing when you wake up in the morning, and you've probably just drained most of your battery in the process. Using the hidden 'Stop Playing' timer, you can choose how long you want the music to play for as you drift off to sleep.
To set up the timer, go to the clock app and then tap 'Timer' in the bottom right of the screen. Choose how long you want your timer to last for (you can choose a matter of minutes or hours if you'd prefer) and then tap 'When Timer Ends'. Here, scroll down to the bottom of the menu and select 'Stop Playing'. Press start on the timer and then begin playing your music from the Music app. When the timer ends, the music will fade to a stop.
This trick will also work for audiobooks and other media.
***Edited to add labels***