I am researching the same thing for my daughter. She's 19 and in college. She sometimes falls asleep but then wakes up again after an hour or two.

First things first, try taking some Melatonin certainly.

So far, from what I've read, stress in your life is the biggest factor you look at, which is remedied by various activities such as exercise, dance, etc. My daughter is in Chicago, so running is out for now but has signed up for a dance class so we'll see what happens there.

Chemical (hormone) changes in your body at 19 are probably the actual culprit here, (and common) so there's not a whole lot of things you can do about it other than what I mentioned above - outside of the melatonin, which is a sleep hormone.

It's been quite a while but I remember having difficulty sleeping in my late teens and early 20s.  Then again, I also remember sleeping real deep and for long hours during the same time frame.  I was probably exhausted from no sleep and my body caught up.