Oversleeping is a reality; some may become conscious early in the morning, but choose to ignore it and fall back asleep. Of couse if you were unemployed and didn't have much to do in the day it wouldn't matter because you can finish a second or third sleep cycle. However, if you're a normal person and have a daily grind, starting a second sleep cycle and then interrupting it may prove to be less than beneficial.
The same thing with eating disorders; it's mostly mental and less physical. I don't doubt that people have different size stomachs and it's not always proportional to their body size.
"Nagi, how do you feel about being 'normal'? Normal means that if you leave something alone, it just remains that way forever. So if you can't stand that, then you have to be "not normal" in some area... - Seiichi Kirima's dying words