I was "camping" two years ago with my son's elementary school, and one of the guys who was chaperoning along with me had apparently been homeless by choice for a 12 month period of time and learned how to sleep outside rather well, in all sorts of weather. Despite the constant rain for the duration of the trip, he slept in a bivvy bag on a pad - no tent!

He told me that he basically lived in the bivvy bag and it was "all you really need"

EDIT: it was one like this:
http://www.campmor.com/outdoor-research-...ci_sku=81533BLU