The First Stage of Sleep is when your body starts to relax. That means that your muscles basically shut down slowly, you stop using them. Thats what your muscles act like when there is no force that they're acting on. Thats also what happens when you're in space. When your muscles relax, you get a feeling of falling down. Not like falling off your chair, but as if the ground just disappeared from your feet and you're falling down eternally. Thats the same feeling of falling that you get in space (zero-g). That feeling of falling is what prompts you to kick out your feet, or swing your arm. It almost always happens right before you fall asleep.