Can physiological sleepiness underlie consciously perceived sleepiness assessed with the Epworth sleepiness scale?

Methods of evaluation of substates of sleep and sleepiness differ in, at least, two respects. Although sleepiness has not been separated from other wake and sleep substates using yes-or-no criteria for sleep scoring, it would be consciously perceived and, therefore, assessed with a questionary. However, such subjective method was challenged by the finding suggesting a disconnect between two most widely used subjective and objective indicators of excessive daytime sleepiness (EDS), a score on the Epworth sleepiness scale (ESS) > 10 and a reduced latency to sleep onset (SOL), respectively. We examined whether these two EDS indicators differ in their association with physiological sleepiness, i.e., the polysomnographic indexes of elevated sleep pressure. In the afternoon hours, polysomnographic recordings were obtained throughout 54 50-min and 56 90-min napping attempts of 27 and 28 university students, respectively. Within some but not all 10-min intervals of the 50- or 90-min naps, each EDS indicator was validated against different objective polysomnographic indexes suggesting an association of EDS with elevated sleep pressure. Significant differences in sleep indexes were found between participants with short and longer SOL, but they disappeared right before the appearance of such differences between participants with higher and lower ESS score (usually at the 4th 10-min interval). This mismatch in timing of appearance of significant differences might be a plausible explanation for the lack of significant association between the ESS and SOL. Therefore, the physiologic underpinnings of the ESS can be uncovered despite such a disconnect between these two EDS indicators. Graphical abstract