ABSTRACT
In the past few decades, human chronobiology research has documented circadian rhythms in a variety of biological and physiological functions, including body temperature, heart rate, and hormone secretions, reflecting regular peaks and declines across the day (Horne & Ostberg, 1976, 1977; Hrushesky, 1994). Circadian rhythms exhibit pronounced effects on important aspects of everyday life, including health and medical treatment (e.g., Hrushesky, 1989, 1994; Smolensky & D'Alonzo, 1993), as well as the ability to adapt to shift work (e.g., Monk, 1986; Moore-Ede & McIntosh, 1993). While extensive research addressing general circadian patterns exists, a far smaller literature concerns the extent to which there are individual differences in these patterns and, in turn, differences in performance at different times of day (e.g., Bodenhausen, 1990; Colquhoun, 1971; Folkard, Knauth, Monk, & Rutenfranz, 1976; Folkard, Weaver, & Wildgruber, 1983). This work has shown that individual patterns of circadian arousal are indeed correlated with performance on a variety of tasks (e.g., efficiency in reacting to stimuli, performing simple arithmetic, engaging in cognitive activity) such that performance peaks at a certain level of circadian arousal, a peak that occurs more or less regularly at a specific point in the day.
