insomnia
"I have been taking melatonin every night for about 3 years and it works good for me! I have never had my sleep problem "diagnosed"....but I can tell you that if I don't take something to MAKE me fall asleep I can easily stay awake for at least 36 hours and then in order to fall asleep I still have to take something! I have tried Tylenol PM and sometimes use pain pills and muscle relaxers (ruptured disc in my neck) and I still need melatonin with any of them! 
I have noticed that I seem to need more than I started out taking."