Margen
In the stage of sleep where we dream most vividly, called REM sleep, your brain is almost as active as when you're awake. Your eyes dart under the lids and your mind builds whole stories. And yet your body doesn't move.
That's no accident: during that stage, the brain disconnects the body's muscles and leaves you practically paralysed. Only the essentials keep going, like the heart, the diaphragm for breathing and the eyes themselves.
It's a brilliant safety mechanism. If you run in a dream, your legs don't actually take off; if you fight, you don't punch the wall.
When that brake fails, people can end up kicking or leaping out of bed while asleep. That it almost never happens to you is proof that, every night, someone bolts the door on your behalf.