Nine young adult male (23-30 years old) paid volunteers were subjected to total sleep deprivation (TSD), after two consecutive nights in the laboratory, for 40 hours (from 0800 hours on the first day to 2400 hours on the following day). Oral temperature (OT), reaction time (RT) in a visual vigilance task, and electroencephalogram (EEG; C3, C4, T3, and T4) while performing the task were recorded every 2 hours during TSD and after recovery sleep. One second of EEG, before target and non-target stimuli for every subject and condition was visually inspected, and artifact-free epochs were Fourier transformed. Absolute power (AP) was calculated for 4-20 Hz (full band) and for theta, alpha 1, alpha 2, and beta 1. Analyses of variance (ANOVAs), with TSD and time-of-day as factors, showed the following significant results. TSD induced an increase in RT and AP of the full band at C3 and C4, of all bands at C3, of theta at T3, and of beta 1 at T4 (p < 0.009 for all comparisons). No time-of-day effects nor interactions were found. OT was not affected by TSD. All variables returned to baseline values after recovery sleep. RT and EEG power showed a linear increase with accumulating hours of wakefulness. The increment in RT also correlated with the increase in EEG power. The results demonstrate that the increment in RT is associated with the increase in AP, particularly in the left central cortex; that the EEG may be used to identify sleepiness; and that EEG during task performance is more sensitive to TSD than during relaxed wakefulness.