Diurnal variations in alpha power density and subjective sleepiness while performing repeated vigilance tasks

Objective: Diurnal variations in EEG activity and subjectively rated sleepi ness while performing repeated vigilance tasks were examined. Methods: Nine diurnally active healthy males underwent repeated vigilance t asks at 08:00, 11:00, 14:00. 17:00 and 20:00 h. An electroencephalogram (EE G) was taken while the subjects performed the tasks with their eyes open. T he alpha power spectra (8.6-13.3 Hz) of EEG was integrated. Subjectively ra ted sleepiness. reaction time and oral temperature were also measured. Results: Significant diurnal variations were found for alpha power, subject ively rated sleepiness and oral temperature. The alpha power was significan tly smaller at 08:00 than at 11:00, 14:00, 17:00 and 20:00 h. The subjectiv ely rated sleepiness was significantly larger at 08:00 than at 11:00, 17:00 and 20:00 h. The diurnal variation in alpha power did not correspond to th at in subjectively rated sleepiness. On the other hand, repeated vigilance tasks increased the alpha power. subjectively rated sleepiness and reaction time at each time of day. The increase in alpha power was significantly gr eater at 11:00 than at 08:00 and 20:00 h. Conclusions: The diurnal variation was found in alpha power while performin g vigilance tasks. Furthermore, the increase in alpha power with repetition of the task depended on the time of day. (C) 2001 Elsevier Science Ireland Ltd. Ail rights reserved.