Time-of-day variations in different measures of sleepiness (MSLT, pupillography, and SSS) and their interrelations

The aim of the present study is to analyze how well physiological measures of sleepiness derived from pupillography and the Multiple Sleep Latency Tes t correlate with a subjective measure, the Stanford Sleepiness Scale (SSS) score, The results are based on data from 12 healthy participants, who unde rwent these tests every 2 hr from 7:00 a.m. until 11:00 p.m. Sleep latencie s were correlated with four different variables derived from pupillography and the SSS score. The results indicate that the physiologically based vari ables correspond very well. This is reflected by similar patterns of time-o f-day variations, a good agreement at the group level, and correlations at the individual level, whereas the SSS shows a quite different pattern of va riation. The two physiological measures of sleepiness seem to reflect the s ame aspect of the level of tonic central nervous activation, which is not c orrelated with the subjective feeling of sleepiness.