Think everyone has their own issue's sleeping sometimes, mine are various, one minute fall asleep in the day, usually from not sleeping for nights on end, (like now), Think iv'e got to many issue's going on in my head, with my health at the moment, so I think I actually know, what's keeping me from sleeping, (over thinking things), then through the day I try to stay awake, not often I fall asleep during day, but I still don't get sleep for days on end, tried hot milk with sugar, tried not looking at TV or computer screens before going to bed , dark room, dim lighted room, generally tried everything.
Personally I think it's what you have on your mind, worries, appointments, generally worrying about something you may have to do, that you really hate, eg, filling in forms, hospital appointments, so in all, my experience is that, the more I fret about sleeping , on top of whatever is on my mind, the worse sleep I have, even though I try to convince myself relax, just have to put this down to, when I need sleep exhausted usually, I will have a good few nights, then hopefully my batteries will recharge.