Power and coherent oscillations distinguish REM sleep, stage 1 and wakefulness.

The objective of this work is to determine differences in spectral power and coherent activity between stage 1 (S1) and REM sleep. The EEG activity of the two sleep stages is almost indistinguishable by visual inspection. Although many efforts have been directed toward understanding the process of falling asleep, little is known about differences in EEG activity between stage 1 (S1) and REM sleep. Polysomnography of 8 healthy young adults from S1, REM sleep and wakefulness was recorded. Spectral power and spectral correlation were obtained for 1-50 Hz. Stage 1 was distinguished (ANOVAs) from REM sleep by lower power in 1-9 Hz, higher power in alpha, beta and gamma, lower interhemispheric correlation in 1-8 Hz and gamma, and higher right correlation in 30-50 Hz. It differed from wakefulness by lower power in 9-50 Hz, but not in 1-8 Hz, or in inter- and intrahemispheric correlation. EEG differences between S1 and REM sleep reside not only in changes in power but also in coherent activity. The different behavior of slow and fast frequencies suggests two different mechanisms involved in the gate into sleep, one implicated in promoting sleep, the thalamo-cortical oscillator mode and the other in reducing alertness involving activation mechanisms. Stage 1 is a mixed state, alertness is already reduced but sleep-promoting mechanisms are not yet fully installed. The EEG differences between these two sleep stages contribute to the understanding of REM sleep and S1 physiology and may be relevant for understanding disorders in falling asleep.