How does he know?

When I am asleep, I am so tired I have know idea what is going on. When Karen comes to bed, I rarely wake-up. But, My son seems to know where I am even when he is asleep. For the last week-an-half he has been really tuned into my moves. Within 15 minutes of me coming home from work, he wakes up from the middle of his nap, calls for me, curls up into my arms, nurses and goes back to sleep. In the mornings, lately, he is awake within 15 minutes of me getting up. Which is hard on Karen because he cries for 20-25 minutes if he see me leave for work, if he wakes up and I am already gone it only bothers him for a moment. Yesterday, when I got off the bed, he started immediately rolling around in his crib (which has one side off and pushed against the bed) with his arms out, then he rolled into our bed, grabbed Karen, realized it was not me and started screaming. I just do not understand how he knows whether I am there or not in his sleep.