Regional differences of the temporal EEG dynamics during the first 30 min of human sleep.

The analysis of the time-course of different EEG bands during the first sleep cycle has up to now taken into account mostly central derivations. In the present study, we assessed the topographical differences of the time-course of different EEG bands during the first 30 min of sleep, by analysing the EEG power from four different scalp locations along the antero-posterior axis. Correlation of EEG bands time courses within and between each derivation has also been evaluated. Delta-theta (1-6.75 Hz) and alpha (7-10.75 Hz) activities exhibited an antero-posterior gradient with maximal power on the frontal lead and increased at all derivations in the first 15 min and 5 min periods, respectively. Then alpha power decreased at all derivations, but the frontal. Sigma EEG power (11-15.75 Hz) showed a coherent behavior over the four derivations, characterized by a steep increase in the first 3-5 min of sleep, followed by a stable decreasing trend. Beta power (16-25.75 Hz) linearly decreased only on the more posterior derivations, abruptly increasing on the frontal lead after a 15 min interval. Correlations between delta-theta and alpha band were higher on the frontal derivation. Moreover, frontal alpha was strongly related to delta-theta activity on all the four derivations, while occipital alpha was not. The negative correlations between delta-theta and beta time courses were very high on all derivations but the frontal one. This study shows the existence of topographical differences in the time-course of different EEG bands during the first 30 min of sleep. The peculiar behavior of the alpha and beta EEG bands over the frontal derivation indicates the need to re-consider the functional role of traditional EEG bands during sleep.