Psychometric evaluation of the Stanford Sleepiness Scale.

Two assumptions underlying the Stanford Sleepiness Scale (SSS) were evaluated: that the descriptors defining each level of the scale are equivalent ways of characterizing a particular level of sleepiness; and that sleepiness, thus measured, is an unidimensional construct. Twenty-four True/False items were derived from the descriptors at each level of the SSS. This revised scale was administered to 340 undergraduates in a questionnaire which also included: the SSS; four visual analogue scales; items identifying the subject's age, sex, and circadian type; and the time of administration. Analyses of the responses indicated that endorsement of items on the revised scale was not consistent with the SSS level endorsed, indicating that the descriptors at each scale level are not equivalent. A principal components analysis revealed two components, tentatively identified as activation and sleepiness, accounting, respectively, for 24.2 and 20.6% of the variance. It was concluded that sleepiness is not an unidimensional construct. Further studies are necessary to elucidate the nature of its components.