Laterality of the sleep onset process: Which hemisphere goes to sleep first?

This study aims to assess whether the hemispheric asymmetry inversion observed in the wake-sleep transition can also be revealed by the latency of inter tapping intervals >= 2.5 s for each hand and the latency of theta burst >= 2.5 s in symmetrical loci of the two hemispheres during the sleep onset process. Data collected from 16 right-handed subjects showed a hemispheric asymmetry in the sleep onset latency with both behavioural and EEG indices. For the first time, a hemispheric asymmetry in the sleep onset latency was found considering a visual analysis of EEG. Results suggest that the hemispheric pattern found during sleep onset can be considered a steady characteristic of the transition from wake to sleep, relatively independent of homeostatic and time of night effects. These results are interpreted as being consistent with the hypothesis concerning an advantage of the right hemisphere in sustaining vigilance. (C) 2007 Elsevier B.V. All rights reserved.