A new perspective on sleepiness

The aim of this investigation was to examine current methods for measuring sleepiness from a new perspective, highlighting different definitions of sleepiness that are being used, inadequacies of the current conceptual framework for thinking about sleep and wakefulness, and the different times-cales used for measuring sleepiness. The currently used methods are briefly described. The results of different tests of sleepiness within the same subjects are then examined, first, using subjectively reported item-scores from the Epworth Sleepiness Scale, and later, using mean sleep latencies in the Multiple Sleep Latency Test and the Maintenance of Wakefulness Test. It is concluded that sleepiness, in the sense of sleep propensity, is not a single characteristic of each person. Measurements of sleepiness involve three sources of variance. First, the person’s average sleep propensity in daily life, a hypothetical construct for which we do not have a direct measure. Second, the somnificity of the person’s posture, activity and situation at the time. This is a manifestation of how the sensory nervous system, and consequently the wake drive in the central nervous system, is influenced in the majority of people by what they are doing. The third source of variance reflects differences in the way some people respond to particular circumstances. The latter influences are both subject-specific and situation-specific. They are presumably learned, and are not very predictable for individual subjects. It is proposed that, to explain these findings, we need to include a new process (Process-A), reflecting afferent inputs to the central nervous system, in any model of sleep and wakefulness.