Melatonin[1][2]
I just have to be careful about taking it too late, if I really need to be up the next morning. I find that when I take melatonin, I really need my full 8 hours (or more) of sleep, or I feel pretty groggy for quite a while after waking up.
[1]: https://en.wikipedia.org/wiki/Melatonin
[2]: http://www.webmd.com/sleep-disorders/tc/melatonin-overview