Even though you feel him moving in there and know he's "still alive"... Lack of movement could be a sign of fetal distress. If you're feeling him move frequently when lying down, though that's a good sign.
When you're up and moving around, the movement helps lull the baby to sleep. When you're just lying there still, they tend to wake. It sucks for us, because it's when *we're* trying to sleep, but it's the truth!!
Isn't the general rule of thumb 15 movements per day? Anything less call the doc immediately? I can't remember. I could've completely pulled that figure out of my you-know-what, but really. If I barely felt the baby move in 48 hours my arse would be in the er! Lol it's best to actually concentrate on it. Like someone else said, sometimes you just don't notice it because you've kind of gotten used to it. But if you start to think he isn't moving enough, stop what you're doing and lay down and count. Just to be save.