A lot of people like Passionflower for this purpose. It's relatively weak in effects but strong enough to really make a difference if you can’t sleep. It doesn't mess up your dreams like some herbs can, and it doesn't leave you with a lingering effect the following day, at least not for me.

Melatonin works for me too but I can always feel slight sedative effects from it the following day which I hate so I rarely ever use it.