Can we feel like being neither alert nor sleepy? The electroencephalographic signature of this subjective sub-state of wake state yields an accurate measure of objective sleepiness level.

Accurate measurement of objective level of sleepiness can have important implications for experimental and field studies of sleep deprived individuals. We proposed to accurately quantify changes in sleepiness level with single electroencephalographic (EEG) measures obtained from EEG spectra consisting of 16 spectral powers within the frequency interval from 1 to 16 Hz. The EEG signal was recorded every other hour from 19:00 of Friday to 19:00 of Sunday in 48 study participants. The differential spectra were calculated for the 1st minute with eyes closed as the differences between EEG spectra for pairs of distinct subjective sub-states (alert, neither alert nor sleepy, sleepy, and very sleepy sub-states scored on the Karolinska Sleepiness Scale as 3, 5, 7, and 9, respectively).The differential spectra were calculated for the sub-samples of those participants who succeeded and failed to succeed in completing all 25 EEG recording sessions (n = 25 and 23, respectively) and for the addition sample of 130 participants deprived from sleep for only one night. Single spectral EEG measures were then calculated by summation of 16 spectral powers weighted by a differential spectrum. The strongest correlation coefficients (0.981, 0.987, and 0.985) were attained between the time courses of subjective and objective measures when data on 130, 23 and 25 participants, respectively, were used for calculation of frequency waiting curve differentiating alert sub-sate either from sleepy sub-state or from neither alert nor sleepy sub-state. We recommended implementation of the proposed objective measure into experimental procedures requiring accurate estimation of objective sleepiness level.