How do cats keep time?

[Sundae]

Cats are known for having a good sense of time.
Diz is always waiting for me when I come home from school. And I mean waiting - Mum will say, "He came down 5 minutes ago, caem into the room to see if you were here and has been on the stairs ever since." Given that my arrival time depends on a number of factors, he's still got it down to approx 15 minutes.

He also starts badgering me from 16.20 onwards, for his fresh meat.
He doesn't always eat it all at once, and always has access to dried food, so I know it's just routine. I try to hold out until the time I have set - 16.30. But if I'm on the PC it's pretty hard to ignore him. Cat on lap, on shoulder, in front of monitor, trying to step on keyboard, switching on priter with his feet - aaaargh!

But I agree that humans have the same ability. I wake up at 07.30 when I need to be at school, and when I'm doing it regularly I'll wake up at this time every day (give or take 15 minutes) - if it's not a school day I turn over and go back to sleep. Over the Christmas break I woke between 09.00-09.15 every day, which is the earliest I can get up and not feel like I'm in my parents' way.