Have you ever suffered a concerned parent trying to convince you to drink your milk, go outside, get eight hours of sleep every night, and stop staring at your phone already? Turns out there is not only a health-related reason to silence your phone while you sleep; it can also keep you from making an ass of yourself.
It sounds like a joke, but sleep texting is a growing problem among teenagers. Try talking to someone while s/he is falling asleep or talking in their sleep; they're not going to remember the conversation. Sleep texting works the same way: you reply to a text and wake up to find you've sent a sketchy message to your friend.
(Okay, so that also has the potential to be hilarious, but do you want to risk it?)
If you need to leave your phone on for emergencies (or if, like me, you use it for an alarm clock) you can install an app to disable email and/or text alerts for a set time period or even when you are in a specific location. Hey, that could also be handy if you don't want to blast your ringtone in class orrrrrrrrrrrrr -- the library ;)