Excerpt from EEG Atlas: Normal Sleep EEG - Stage I

Synonyms, Key Words, and Related Terms: EEG atlas, normal sleep EEG, stage I sleep, drowsiness, presleep, sleep stages, rapid eye movement sleep, REM sleep, nonrapid eye movement sleep, NREM sleep, slow rolling eye movements, SREMs, attenuation (drop out) of the alpha rhythm, central or frontocentral theta activity, enhanced beta activity, positive occipital sharp transients of sleep, POSTS, vertex sharp transients, hypnagogic hypersynchrony, EEG waveforms

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Loomis provided the earliest detailed description of various stages of sleep in the mid-1930s, and in the early 1950s Aserinsky and Kleitman identified rapid eye movement (REM) sleep. Sleep generally is divided in two broad types: nonrapid eye movement (NREM) sleep and REM sleep. On the basis of EEG changes, NREM is divided further into 4 stages (stage I, stage II, stage III, stage IV). NREM and REM occur in alternating cycles, each lasting approximately 90-100 minutes, with a total of 4-6 cycles. In general, in the healthy young adult NREM sleep accounts for 75-90% of sleep time (3-5% stage I, 50-60% stage II, and 10-20% stages III and IV). REM sleep accounts for 10-25% of sleep time.

Stage I sleep also is referred to as drowsiness or presleep and is the first or earliest stage of sleep. Representative EEG waveforms are shown in Images 1-5.

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