Circadian rhythms, cyclic fluctuations in many physiological and psychological functions, are thought to influence adjustment to shiftwork. A widely acknowledged individual difference in circadian rhythms, commonly called morningness, indicates preferences associated with morning or evening activities. Various self-report instruments have been developed to measure morningness, although little measurement data have been published for these scales. Because morningness scales are being used to select workers for night shiftwork, psychometric evaluations of these scales are needed. Psychometric assessments of undergraduate responses (N = 501) on three widely used scales indicate internal (interitem) measurement deficiencies in all three. Therefore, a 13-item scale was developed that distills the best items from two of these scales. Relationships between the new composite scale and external criteria are comparable with or stronger than similar relationships between the published scales and external criteria.

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