“Microcyclic” Changes in Brain Bioelectrical Activity at Different Stages of Natural Sleep in Humans

We report here data on a number of systematically recorded phasic EEG phenomena during the sleep–waking cycle, along with data on the heterogeneity of the EEG stages of sleep forming sequential sleep macrocycles in humans. These transient EEG changes quite often showed a tendency to periodicity, especially at the initial stages of sleep and in transitional states; characteristic sleep microcycles were observed. An attempt to identify the microstructures of EEG sleep stages during the sleep–waking cycle was made by identifying seven clusters reflecting changes in the biopotential field of the brain represented in n-dimensional factorial space. Most sleep stages were found to consist of three or four clusters, though some sleep periods (especially Loomis stage B and 1REM) were more homogeneous in structure. It is suggested that this heterogeneity of the microstructure of the spatial organization of oscillations in brain biopotentials in particular sleep stages reflects the dynamics of neurophysiological processes, promoting more effective performance of the repair and homeostatic functions of sleep.