ABSTRACT
What is sleep? Sleep scientists might define sleep as a period of behavioral quiescence and non-responsiveness to the environment that is electroencephalographically, physiologically, and behaviorally distinct from the waking state. Sleep is divided into two states, rapid-eye-movement (REM, or “paradoxical” sleep in animals) and non-REM (NREM) sleep that are also electroencephalographically, physiologically, and behaviorally distinct from one another. NREM sleep is further subdivided into four stages 1-4 (or I-IV), corresponding to the depth of sleep, and the presence of specific electrophysiologic markers.
