TIME OF DAY, SPEED OF RESPONSE, ALERTNESS AND FATIGUE

Andrew P. Smith, PhD*

ABSTRACT

Background: Human performance changes over the course of the day. The profile of the time-of-day effect depends on the type of task carried out. Responses in reaction time tasks are faster later in the day, and this may reflect a speed-error trade-off or changes in the speed of encoding new information. Alertness peaks in the late morning and is reduced after task performance. The present study examined whether changes in reaction time reflected differences in fatigue caused by performing the task in the morning and afternoon. Method: Two hundred and seventy two staff or students (159 females, 113 males; mean age 35.4 years, age range 17-65 years) from Cardiff University participated in the study. They completed the tasks in either the late morning (11.00-13.00) or afternoon (16.00-18.00). Participants carried out a focused attention two-choice reaction time task and a categoric search task. They also rated their alertness before and after the tasks. The main outcomes of interest were mean reaction times and alertness. Secondary outcomes were selective attention measures, errors, lapses of attention and the speed of encoding new information. Results: Reaction times in both tasks and for specific conditions in the tasks were faster later in the day. Alertness was higher in the morning. Performing the tasks induced fatigue, but this did not vary over the day. Conclusion: The present study confirmed that alertness is higher in the late morning than in the late afternoon. In addition, alertness is reduced by task performance. Reaction times were quicker later in the day, but these did not reflect diurnal variation in the fatigue induced by performing the tasks.

Keywords: Background: Human performance changes over the course of the day. The profile of the time-of-day effect depends on the type of task carried out. Responses in reaction time tasks are faster later in the day, and this may reflect a speed-error trade-off or