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sleep stage episode

A sleep stage interval that represents the stage in a non-REM-REM sleep cycle; easiest to comprehend in relation to REM sleep, which is a homogeneous stage, i.e., the fourth REM sleep episode is in the fourth sleep cycle (unless a prior REM episode was skipped). If one interval of REM sleep is separated from another by more than 20 minutes, they constitute separate REM sleep episodes (and are in separate sleep cycles); a sleep stage episode may be of any duration.

Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann