I think thats kind of asking like what makes you blink, one your eyes need to so we'll chalk that up to your body going to long without blinking (or in the sleep case, to long with sleeping), something may scare you causing a blink (or wake up), and a general loud stimulus (if you've gone to a really loud concert, you may blink every time they hit the bass drum, but it doesn't scare you), so likewise hearing construction my not scare you but it will wake you up. So thats my answer, a multitude of things! I'd say the most natural, however is due to our circadian rhythm, our body has received the sleep it needs so it wakes up.
-kilon828