Diurnal variations in the waking EEG: comparisons with sleep latencies andsubjective alertness

Daytime measures of sleep latency and subjective alertness do not correlate with one another, suggesting that they assess different aspects of alertne ss. In addition, their typical diurnal variations show very different time courses. Quantitative analysis of the waking electroencephalogram (EEG) has been proposed as an objective measure of alertness, but it is not clear ho w it compares with other measures. In this study, the waking EEG was measur ed in the daytime to determine the presence of diurnal variations in the ac tivity of standard frequency bands and to compare these variations with the temporal patterns typical of sleep propensity and subjective alertness. Al ertness was evaluated in four men and 12 women, aged 19-33 y. Assessments w ere conducted every 2 h, from 10.00 to 24.00, in the following order: a vis ual analogue scale of alertness, a waking EEG recording and a sleep latency test. The waking EEG was recorded with eyes open. For each recording sessi on, 32-60 s of artefact-free signals were selected from the C3/A2 derivatio n, then subjected to amplitude spectral analysis. Four EEG frequency bands showed significant diurnal variations: delta, theta, sigma and beta1. None of these variations showed a significant correlation with the temporal patt erns of sleep latencies or subjective alertness. At the individual level, h owever, theta band activity increased when subjective alertness decreased, suggesting that the theta band can be used to monitor variations in alertne ss in a given individual, even at the moderate levels of sleepiness experie nced during the daytime.