How Do I Schedule My Iphone 3g Sounds To Automatically Mute Or Go Silent At Night When I Sleep

I would like to schedule my iphone to mute or go silent during sleeping hours so that notifications don't wake me up. I still need calls to come through in case of an emergency and I still need the alarm clock to work so I'm not late for work. Is there any way to do this? Maybe an app that I just can't find.