No-one can actually sleep that much, unless very ill. He'd become unable to sleep at night, or during the day - at some time anyway. He could be resting and dozing but, even when quite wiped out, that gets boring after a while. Is he just withdrawing from you and your activity, rather than actually sleeping?
Does he actually go to bed and sleep - no tv, no radio, no laptop, no phone?
Who are all these men who nap? I certainly haven't met them in my working life!
It is possible to get into very bad sleeping habits that mean you take up a lot of time getting poor quality sleep.
It is also true that some people get to point of just needing to sleep and conk out more suddenly than others. DP is a morning person and just conks out at night, though when he wakes up he's fully awake. He does sometimes fall asleep during the day. I'm an evening person and have much more stamina to carry on tired, though, especially if mentally tired, I do sometimes get to a 'just need sleep, now' point. I very rarely sleep in the day and find it very hard to let go of some mental alertness e.g. I have never slept through my train station.