Different phase relationships between EEG frequency bands during NREM and REM sleep

Phase relationships between distinct frequency bands of the sleep electroencephalogram (EEG) were studied in healthy subjects using cross-correlation coefficients, both over the entire night and separately for nonrapid eye movement (NREM) and rapid eye movement (REM) sleep. Over the entire night, a large positive correlation developed within high- and low-frequency bands, while a negative correlation emerged between low- and high-frequency bands, reflecting their reciprocal temporal course. More detailed analysis revealed different phase relationships during NREM and REM sleep. Findings during NREM were similar to the entire night. However, during REM, a large increase of the correlation between low- and high-frequency bands occurred compared to NREM, even with a change of sign. The results indicate the complexity of the interrelation of the different frequency bands depending on sleep state, with low- and high-frequency bands being out of phase during NREM and in phase during REM sleep.