no offense to anyone, but the idea that someone might need less sleep because their day is less strenuous demonstrates a belief that sleep is about rest. it isn't. for that matter, it isn't about the amount of fat your body has either (not that not getting enough sleep won't negatively effect you such that you might gain fat). actually, we really aren't sure why we sleep. there are a lot of hypotheses out there, but, right now, we just don't know what sleep is really about. what we do know is that most of the repair to our bodies is taken care of while we sleep. outside of that we're still figuring it out.
whatever sleep is about, it isn't about rest. no matter how much someone rests they still need sleep, or they develop mental deficiencies.