My armchair-biologist answer is that when forcing yourself to stay awake through the sleepy periods, you're signaling to the body that there is something dangerous happening and that you need to stay alert and that makes it harder to sleep. For me it usually helps if I can sneak in a nap during the day, makes me more relaxed in the evening.

Don't know but probably not anthropology

Caffeine doesn't work like that for some people. I could drink a cup of coffee 2 hours before going to bed and it wouldn't affect my sleep at all. Although currently I am limiting myself to just one cup only for a day, because my blood pressure have gone crazy one time and I thought I would collapse while commuting by bus (drank 5 cups of coffee before 5 PM like an idiot).