My wife needs 8-9 hours, whereas I can get by just fine with 5-6. She is a very light sleeper who will wake up 3-4 times every night; I can sleep through almost any noise, and never get up in the middle of the night. My son takes after me -- he seems to need significantly less sleep than other kids his age, but he's an extraordinarily deep sleeper.
Anecdotal, but I think that sleep depth might have something to do with it.
Anecdotal, but I think that sleep depth might have something to do with it.