Brief and distinct empirical sleepiness and fatigue scales

Sleepiness and fatigue are conceptually distinct but pervasively confounded in research, measurement instruments, clinical settings, and everyday spoken language. The purpose of the present study was to construct two scales that represent unconfounded measures of sleepiness and fatigue, using widely used questionnaires.

Four questionnaires purporting to measure sleepiness [Stanford Sleepiness Scale (SSS); Epworth Sleepiness Scale (ESS)] or fatigue [Fatigue Severity Scale (FSS); Chalder Fatigue Scale (CFS)] were administered, as well as a battery measuring sleep, psychological, and health functioning variables, to three samples: 19 individuals with chronic fatigue syndrome, 14 with narcolepsy, and 11 normal control subjects.

Analyses revealed two distinct sets of items (six sleepiness and three fatigue items) that were combined into two scales. These newly formed scales are only minimally correlated and represent separate constructs that have reasonably distinctive patterns of association. Findings were replicated and validated in a sample of 128 older individuals complaining of daytime sleepiness and/or fatigue.

We conclude that (a) it is possible to derive empirically distinct sleepiness and fatigue scales from existing, commonly used self-report instruments, (b) the Empirical Sleepiness Scale is limited to the experience of daytime sleep tendency, while (c) the Empirical Fatigue Scale is associated more broadly with insomnia, psychological maladjustment, and poorer perceived health function. The important clinical implication of the new Empirical Sleepiness and Fatigue Scales is in the ability to identify "sleepiness which is not fatigue," a construct closely related to primary sleep disorders, such as sleep apnea/hypopnea syndrome, for which there is both available and effective treatment.