I think it's a stress thing as I never used to do it but ever since I've been home for the summer I've started doing it, apparently I can get really bitchy in what I say as well so if it carries on when I go back to uni in my new house with very thin walls I could have some nice silences awaiting me over breakfast
Think it's just one of those things, just try and write down everything on your mind before you go to sleep so hopefully your brain will be less inclined to articulate them out loud in your sleep, maybe?
Or take a sleeping pill? Seems a bit extreme though. I don't know, I think it's one of those things you just grow out of and either do or don't, quite randomly.
I used to scream at my mum in my sleep. And I once punched her in the face (also whilst sleeping)