get into a routine. it's like eating at fucked up times during the day and wondering why you aren't hungry when 6:00 rolls around. get into the habit of going to bed at the same time every night, and wake up at the same time every day. your body will get used to it.
and if your internal clock is all fucked up, go to bed late one night and wake yourself up at 6am the next morning. i guarantee you'll be tired the next night. sometimes that's what it takes to reset yourself.
and dont think too much as youre falling asleep. sometimes falling asleep requires active effort. you cant just lay there and expect to sleep. you need to try to sleep. i'll get myself into a comfortable position and start focusing on my breathing, or ill think of the different parts of my body and try to relax them as much as possible. ill wiggle my toes a bit and then relax, and then ill flex my legs and relax, and so on. really helps mellow you out and gives you that feeling where you're just sinking into your bed with nothing on your mind but how relaxed you are and letting yourself doze off.