It’s very possible I’m the only person who didn’t know this, but just in case: you can go to the Clock app and set a timer, and then under “When Timer Ends,” instead of picking an alarm tone, scroll all the way down and set it to “Stop Playing.” System-wide sleep timer! As someone who sometimes likes to fall asleep listening to podcasts or The Office episodes, this is a game-changer. Thank you, five-year-old Reddit thread, for solving this for me.