Sometimes I am playing a game while watching some streaming, and then fall asleep in the middle of it. Any pending downloads won't start until I manually resume it. I already have some applications like this, but I automatically turn off my PC when I fall asleep to prevent wasting any electricity. Is there some way to make downloads automatically resume after being away for a certain amount of time?
You can allow Steam to background download games on a per-game basis via the properties menu of each game.
If you disable this on all games via Steam settings, then the downloads should resume as soon as you quit a game.
Here's a question that has a lot more details about these settings: How do I continue downloading a game in Steam while playing?