Excerpt from EEG Atlas: Normal Sleep EEG - Stage II

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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 II is the predominant sleep stage during a normal night's sleep. The distinct and principal EEG criterion to establish stage II sleep is the appearance of sleep spindles or K complexes. The presence of sleep spindles is necessary and sufficient to define stage II sleep. Another characteristic finding of stage II sleep is the appearance of K complexes, but since K complexes typically are associated with a spindle, spindles are the defining features of stage II sleep. Except for slow rolling eye movements, all patterns described under stage I persist in stage II sleep (see EEG Atlas: Normal Sleep EEG – Stage I).

Representative examples of the waveforms described here are shown in Images 1-6.

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Please click here to view the full topic text: EEG Atlas: Normal Sleep EEG - Stage II