Channels can have its playback set to stop after a certain amount of time. This serves as a sleep timer.
Set the Sleep Timer
While watching live TV or a recording on Apple TV, swipe down to get the Info Bar and choose the Options tab. Select the Sleep Timer option to set how long it is.
In iOS, while watching live TV or a recording, use the More button in the navigation bar to get to the Sleep Timer option.
How it works
Once the timer has ended, the playback will stop, allowing your device to sleep. The sleep timer will persist between channel changes when Channel Surfing or changing via the Quick Guide. When you leave the player to go back to the main menu, the Sleep Timer will reset.