Psychometric evaluation of daytime sleepiness and nocturnal sleep onset scales in a representative community sample

Background: The public health importance of daytime sleepiness as a risk fa ctor for accidents, interpersonal problems, and decreased productivity has been recognized. However, epidemiologic research on this topic has been lim ited by the reliance on laboratory measures (i.e., the Multiple Sleep Laten cy Test-MSLT). Two scales, daytime sleepiness and nocturnal sleep onset, ha ve been identified from the self-report Sleep-Wake Activity Inventory (SWAI ) in a clinic sample and validated against the MSLT. This study evaluates t he replicability of the two scales in a population sample and assesses pote ntial thresholds in scale scores that distinguish normal from pathologic le vels of daytime sleepiness and difficulty falling asleep. Methods: The sample consisted of 2181 subjects 18-45 years old in the Detro it metropolitan area. All sleep characteristic information covered the 2 we eks prior to interview, Split-half sample factor analyses were conducted to assess replicability of the results. Distribution of scale scares and thei r relation to construct validity variables were used to evaluate possible t hresholds. Results: A two-factor model appeared to best account for the variation amon g the 12 items from the SWAI. The two factors accounted for 50% of the vari ance in both split-half sample analyses. The revised eight-item daytime sle epiness and two-item nocturnal sleep onset scales showed good and fair inte rnal consistency respectively across both split-half samples. There appeare d to be a "natural break" in daytime sleepiness scale scores that was assoc iated with a substantial and consistent change in number of hours slept. No breaks appeared in nocturnal sleep onset scores. Conclusions: This study replicated the results of the clinic-based study an d suggested a potentially useful diagnostic threshold for self-report exces sive daytime sleepiness, Epidemiology of sleep depends on the ability to mo ve from the laboratory to population surveys in reliable and valid ways, De velopment of self-report is a step in that direction. (C) 1999 Society of B iological Psychiatry.