Interaction of age with time of day and mental load in different cognitive tasks.

Extensive research has demonstrated that shiftwork produces deleterious effects on health because of the desynchrony it induces in the biological clock. The problem is even more crucial for older workers who present, in addition, various decrements in their cognitive functioning, particularly on attention and memory. The present study assessed whether age was related to task complexity as a function of time of day and time-on-task in a rapid rotating work-rest schedule. 24 subjects (12 juniors: 20-30 years and 12 seniors: 50-60 years) performed either a simple task (visual discrimination) or a complex task (descending subtraction) on three different moments of the day simulating the main shifts (morning, evening, and night). Analysis indicated that an age effect was only present on the more complex task, which was demanding in attentional resources and memory load. Seniors had no deficit in performance on the simple task compared to juniors. The effect of time of day was restricted to the simple task for both age groups. However, some differential strategies appear to distinguish juniors and seniors, specifically on accuracy during the night, suggesting that subjects of different ages cope with cognitive tasks in different ways and that perhaps some adverse effects apparently associated with aging could be counteracted by efficient strategies, but not others.