Why is it that when i sleep I do crazy things?

I used to spend the night at my friends house all the time and he would tell me that at night i do crazy things such as: Scream profanity, pick up things and throw them, and even punch and kick people in the room. I would never remember doing any of these things in the morning so I didn't believe him. Now that I have moved to college, my roommate is telling me similar things, and he has never met my friend so this isn't some practical joke. If anybody knows whats going on or has experienced some of the same things i would love to know.