Performance Fluctuations of Low and High Demanding Tasks: Diurnal Variations in Resources

Performance fluctuates systematically during the day, which fluctuation is referred to as ‘diurnal variation’. Performance is maximal at midday and minimal in the morning and at night . However, a continuous decline over the waking day is sometimes found . If multiple testing sessions are conducted during mid-afternoon, a post-lunch dip is sometimes found . In order to clarify the task differences in peak performance, investigators have proposed that performance peaks may be explained in terms of resources. The lower the vigilance level, the less resources might be available . Tasks that require a high amount of resources will consequently be the first tasks to yield worse overall performance. Only when resources have declined to a large extent will task performance get worse on simple, less demanding tasks. Moreover, demanding tasks should therefore also show diurnal variation to a greater extent. Task performance is considered to reflect vigilance level , but vigilance can be determined with other measures, as well. Diurnal trends have been reported for EEG parameters . Subjective alertness follows a comparable pattern and has been shown to correlate with task performance . Furthermore, subjective alertness has been reported as a marker for resource availability . Therefore, vigilance level and resources appear to be closely related. The level of vigilance is determined by task performance and subjective alertness and thus used in a descriptive manner. We investigated diurnal variation in vigilance and tested the hypothesis that it is the availability of resources that determines the timing of the peak in performance. We used the relatively simple SART, which is considered to measure vigilance , and a revised version of the SART. The adapted version of the SART places a great demand on memory and thus on resources. We predicted that both subjective alertness and task performance would show the inverted u-shaped pattern of diurnal variation. Performance on the demanding ‘memory’ SART is expected to show an earlier peak and to display more variation during the day than the simple SART.