In sleep mode your computer's processor shuts down, and your RAM is the only 'live' component (hence the minimal battery usage). Of course, nothing can go on if your processor is turned off! GameXplain recently released a video in which they test if the Switch will download games faster in Sleep Mode. While some complications arose in figuring out just when the data had actually finished downloading, the results seem to show games being downloaded about 15% faster in Sleep Mode compared to the consoles normal state.
Well, as others have said, it goes into a standby mode. However, you can turn that off in options if you are like me and want it to power all the way off. I was having issues with it acting strange from time to time with not powering it all the way off. Games that were still running would freeze up and stuff.Oh this is an old thread.
If your game is stuck at 20% downloading try canceling it and redownloading it. Or ur internet is just terrible.
I have bad internet but games can easily finish downloading overnight.