One wierd thing I've noticed is that the sound of a clock is very important to me. If I change clocks, and the alarm sound is different, it won't wake me up.....almost like the sound is build into my subconcious, and it wakes me up because I know the sound means wake up.
I never sleep in quiet....I hate quiet, but even if my clock has less decibals than the sound going on....it wakes me from memory or something. Everytime I buy a new clock.....it won't wake me up very well until I get used to it......strange I think. I have even had new clock alarms enter my dreams, but not wake me up....one time I was dreaming, and the dream was me trying to figure out where this sound was coming from.
On a side note......it seems the sounds around me always get in my dreams....I sleep with the bedroom TV on ALWAYS because I like sound when I sleep.........whatever show is on often enters my dreams, so with this is mind.....It makes sense why a new clock with a different sound doesn't wake me up.....it just enters into my mind and dreams instead.
I can sleep through a war, a flood, and a marching band outside playing for a guy mowing his grass, but my alarm clock's sound is burned into my brain, and always instantly wakes me.....I do hit snooze a few times, but it wakes me every time even if other sounds don't that are MUCH louder......strange I think.