Until the 1950s, doctors and other researchers believed that sleep was a single state distinguishable only from waking. However, we now know that sleep itself is divided into two distinctly different states: REM (pronounced as a single word, “rem”), or “rapid-eye-movement” sleep, and non-REM sleep. During non-REM sleep you lie quietly, with a regular heart rate and breathing pattern; it is probably closest to what we usually think of as “sleep,” and it provides most of sleep’s restorative properties. There is very little dreaming in this state, if any, although thoughtlike processes may
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