Well for me it has been a number of things. Diet, exercise, environment, et cetera.
First off, I don't recommend the use of sleep aids or medicine. 90% of illnesses can be cured and/or prevented without medication. With that being said...
- Is your sleep schedule consistent?
- Do you give yourself an hour to rest your eyes from bright screens? (computer screens, TVs, cell phone, and so on) These devices can stimulate your brain and keep you awake for longer than you should.
- While we're on the topic of brightness, how dark is it in your room? If it isn't pitch black, it's possible that you still might be stimulated from even the smallest lights (e.g. LED from an alarm clock can even keep you awake). If it isn't pitch black, you can do what I do since I have way way WAY too many electronic devices in my room to cover all the LEDs for by putting on an eyemask/sleepmask/blindfold. Get the ones that don't press on your eyeballs, as those can be bothersome as well.