Performance and circadian rhythms.

Physiological studies frequently neglect effects due to individual differences, thus inflating the error variance term. Psychological determinants of endogenous arousal are examined in association with somatic functioning and circadian rhythms. Frontalis EMG and heart rate did not appear to display clear phasic circadian rhythms. The subjective perception of activation displayed a cubic trend, independent of sex, exhibiting two peaks, at 08.00-11.00 and 17.00-20.00. Peak performance on a choice reaction time task depended very much on whether it was executed in a free response or experimenter determined tempo condition, indicating the importance of stipulating the task demand characteristics. There was a certain stability in extraverts (cortically less aroused) across periods throughout the day with respect to physiological measures; the same was not true for introverts, who exhibited a complex interaction with time of day - stable introverts possessing higher cardiac rates than unstables in the earlier part of the day, the position reversing itself in the late evening. Personality variables can exercise moderating influences on physiological activity associated with performance.