Have you ever wondered why you don't physically act out your dreams? It's because a biochemical switch, tripped as you enter the REM phase, literally blocks the brain's ability to consciously order your muscles to move. If this switch fails to work, all sorts of REM-related oddities can ensue: People might walk, talk, eat or do violence while sleeping. Victims of a condition known as night terrors literally live out their nightmares. And remember my own story about waking up paralyzed? That's what happens when the switch does its work too well, and the mind wakes up before the body regains its ability to move.