Topographical and temporal patterns of brain activity during the transition from wakefulness to sleep.

Changes in electroencephalographic (EEG) spectral power, coherence and frequency were examined for the last minute of wakefulness and the first minute of sleep via topographical mapping. Data were also analyzed across sequential 1-minute samples of wake, stage 1 and stage 2 sleep. Not all brain regions exhibited the same EEG changes during the transition and not all brain regions were found to change at the same time. Brain sites closest to the midline (e.g. F4, C4, P4, O2) showed significant changes in EEG power (increases in theta and decreases in alpha power) during the transition to sleep, whereas brain sites most lateral to the midline (e.g. Fp2, F8, T4) showed little change. Decreases in alpha coherence were observed from wakefulness to sleep for brain site comparisons furthest away from each other (e.g. T3 vs. T4, T5 vs. T6, F7 vs. F8, F3 vs. O1, F4 vs. O2). Spectral analysis of EEG activity revealed that the time of significant change in EEG power varies among brain regions. Decreases in alpha power continued to occur later into the transition period for the posterior regions of the brain (O2, P4).