Half Dome can be done in one day, absolutely. That's how I've always done it. We usually leave at about 6am so as to get up the steepest bit before it heats up too much. Then the rest is a pretty casual hike, with lots of rest stops and such. Then we usually spend about an hour at the top, and head back down. I think it usually works out that we get back to the trailhead at 6pm or so.
And Emily, there are places that do not require sleeping outside
