Cognitive Performance in Middle-aged Adults as a Function of Time of Day and Task Load

The aim of this study was to explore the influence of task complexity and time of day on middleaged shift-workers. Two groups of twelve subjects participated to this experiment: one Junior group (aged 20-30) and one Senior group (aged 50-60). Performance was assessed in two tasks involving different cognitive load: a simple visual discrimination task and a complex mental arithmetic test. The former was considered as an immediate processing task requiring little memory and attention, whereas the latter required a high mental load in working memory and attention. The subjects were tested in both tasks at three different times of the day simulating the main shifts: morning, evening and night. In each task, the test session lasted 60 minutes, divided into two 30-minute periods separated by a 20-minute rest. The results showed slower mental processing in both tasks in Seniors revealed by increased reaction time. Accuracy was also decreased, but only in the complex arithmetic test with an increase in error rate which was not observed in the simple perceptual task. Mental speed in the calculation task was decreased in both groups during the night session, but there was no time-of-day effect in the simple task. The time course of accuracy during the second period of the night session showed increased errors in Juniors but a decreasing trend in Seniors. Altogether, these results demonstrate that performance deficits due to age appear rather early in the lifespan, particularly when tasks involve high memory and attention load. However, the opposite profiles between Seniors and Juniors on accuracy during the night suggest a possible difference in strategies between young and old subjects which could help Seniors to partly counteract their deficits due to age.