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The short answer Sleep is not one state. It moves through four distinct stages (three non-REM stages plus REM) in cycles of roughly ninety minutes, and the mix changes as the night goes on. Slow-wave sleep is concentrated in the first part of the night; REM periods get longer towards morning. That structure is what "sleep architecture" refers to, and it is the reason unconsciousness and sleep are not the same measurement. A sedative can produce the first without reproducing the second. This arti …