musing about the clock inside human body.

hi. anyone knows how our internal clock knows what time is it? i mean, when I say to myself, "wake up at 5am tomorrow", i'd actually wake up at 5 (+/- 5 minutes). i mean, does our brain keeping tab on what time is it, or do i wake up unknowingly and look at the clock and sleep again or something? i cant tell when i'm awake what time is it without see a clock, tho. btw that is a common knowledge, right? that when we tell ourselves to wake up at certain hour, we will? some of my friends can do it too.