Varies, sometimes it's a dog barking outside, or my dog barking at someone who isn't there inside, since he's friggin' senile, but more often than not the first sound I hear is the one I make, whether it's rustling the sheets or whatever. I work nights and as such have accustomed myself to be a very heavy sleeper. Don't even block out the light in my bedroom anymore, I could probably sleep through a tornado (I've actually slept through a mild earthquake.) I don't use an alarm clock since they don't work, although usually if I'm about to oversleep my mind wakes me up. And I mean that. I'll actually have a dream of myself telling me to wake up, and I will.