Deep sleep and REM sleep get lumped together as "good sleep," but they're almost opposite states — one is the quietest your brain gets all night, the other is nearly as active as being awake. In this episode we pull the two apart: how we stumbled onto each of them, what's happening chemically while they run, and why one of them quietly shrinks as we age while the other holds steadier. In this episode we will: Break down what's actually happening in slow-wave sleep versus REM — brainwaves, mus...