Assessing subjective daytime sleepiness: an internal state versus behavior approach.

The authors' purpose in the current study was to apply P. J. Lang's (1968, 1971, 1985) theory of multiple systems of emotional response to the study of subjective sleepiness. A total of 274 participants completed the Stanford Sleepiness Scale (SSS), the Profile of Mood States (POMS), three sleepiness-related Visual Analogue Scales (VAS), and the Epworth Sleepiness Scale (ESS). The authors completed correlations and factor analysis of the 4 sleepiness measures. The SSS, POMS, and VAS measures were better correlated with each other than with the ESS. Similarly, the SSS, POMS, and VAS measures loaded highly onto one factor, whereas the ESS loaded highly onto a separate factor. These results indicated that the ESS measured a different aspect of subjective sleepiness than the SSS, POMS, or VAS. According to Lang's emotional responses theory, the ESS assessed a behavioral component of sleepiness, and the SSS, POMS, and VAS measures assessed an internal state related to sleepiness.