Quick Answer
While your sleep story is playing, tap the Timer (or half-moon icon) on the player screen and choose how long you would like the audio to play.
Method 1: Using the Timer Icon
Start playing your sleep story
Tap the Timer icon at the bottom of your player screen
Select the length of time you would like the story to play
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Method 2: Using the Half-Moon Icon
While on the meditation screen, tap the half-moon icon at the bottom left
Choose when you would like the audio to stop
Tap HERE to learn how to "stream/repeat" your meditation to play longer.