Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
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sleep stage 3
A stage of non-REM sleep defined by at least 20% and not more than 50% of the episode consisting of EEG waves less than 2 Hz and more than 75 µV (high-amplitude delta waves); a "delta" sleep stage. With stage 4, it constitutes "deep" non-REM sleep, so called slow wave sleep (SWS); often combined with stage 4 into non-REM sleep stage 3/4 because of the lack of documented physiologic differences between the two, appears usually only in the first third of the sleep episode; usually comprises 4-6% of total sleep time.