I think you are tying together two things that may be unrelated. I may be wrong in my opinion, but generally speaking general health is usually related to genetics and lifestyle. Sleep is but one component of that lifestyle. Not to say that NOT getting the right amount of sleep can't hurt you. It certainly can. But that in and of it's self is rarely the deciding factor in health levels.
I'm not sure I agree with Seth on the sleep thing. I do think a lot of people over sleep and under sleep. A nap is a good thing too. I believe that you have to obtain several sets of the sleep cycles in order to optimise the benefits of sleep. Every individual is different, so you need to listen to cues from your body.
It seems to me that your drowsy moments are fairly close to your meal times. Is it possible that your body is just trying to efficiently digest, so it pulls the blood toward the belly, thus depriving extremities. That being said, the slow down of peripheral blood circulation might trigger the brain to shut down (or get sleepy).
Just my thoughts