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sleep stage 1

A stage of non-REM sleep that occurs at sleep onset or that follows arousal from sleep stages 2, 3, 4, or REM. It consists of a relatively low-voltage EEG with mixed frequency, mainly theta activity and alpha activity of less than 50% of the scoring epoch. It contains EEG vertex waves and slow rolling eye movements; no sleep spindles, K complexes, or REMs. Stage 1 normally represents 4-5% of the major sleep episode.
Credit

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