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Variations in period-analysed EEG asymmetry in REM and NREM sleep.

Monopolar EEG was recorded from lateral frontal and parietal sites with linked ear reference during sleep in 24 adults. Electrode placement followed the 10-20 International system. EEG was quantified using digital period analyses. The absolute difference in interhemispheric EEG parameters was compared for Stage 2, REM, and a slow wave sleep measure. The absolute difference measures reflect the degree of symmetry or asymmetry, regardless of the hemisphere of origin. Theta and delta activity in slow wave sleep was more asymmetrical than in either Stage 2 or REM. REM sleep was associated with the smallest asymmetries. These results do not support a right hemisphere REM, left hemisphere NREM relationship. Rather they suggest that REM sleep is associated with relative hemispheric symmetry whereas asymmetries are most prominent in slow wave sleep. Stage 2 sleep was significantly less asymmetrical than slow wave on a number of theta and delta measures. The significant differences between slow wave and Stage 2 sleep may denote functional differences within NREM sleep stages.