-Cognitive performance is affected by an individual ' s characteristics and the environment, as well as by the nature of the task and the amount of practice at it. Mental performance tests range in complexity and include subjective estimates of mood, simple objective tests (reaction time), and measures of complex performance that require decisions to be made and priorities set. Mental performance tasks show 2 components, a circadian rhythm and the effects of time awake. The circadian rhythm is in phase with the rhythm of core temperature and there is evidence for a causal link. Increasing time awake results in performance deterioration and is attributed to fatigue. The relative contribution of these 2 components depends upon the task under consideration; simple tasks generally show smaller effects due to increasing time awake. These contributions have been assessed by constant routines and forced desynchronization protocols and have formed the basis of several mathematical models that attempt to predict performance in a variety of fi eld conditions. Mental performance is negatively affected by sleep loss; although short naps are benefi cial, sleep inertia limits their value immediately after waking. The processes involved in cognition include attention (tonic and phasic alertness, and selective and sustained attention), working memory (phonological, used for speech, reading, and writing; and visuospatial, used for spatial processing, drawing, and mathematics), and executive function (initiative, decision making, and problem solving). These processes are illuminated by analysis of the regions of the brain involved, the presence of circadian rhythmicity, and the effects of sleep loss. The results from such laboratory-and fi eld-based observations are relevant to the issue of learning in schoolchildren and lead to suggestions for improving their performance.
INTRODUCTIONThere is a large body of research that considers biological factors that are associated with accidents or errors in the workplace, the aim being to develop strategies that minimize such occurrences. This approach has been complemented by laboratory-based studies that investigate the factors that affect mental performance, the basic argument being that poor performance is more likely to result in errors which, if uncorrected, lead to accidents (reviewed in Åkerstedt, 1995, 2007Dinges, 1995;Folkard, 1990;Horne & Reyner, 1995;Waterhouse, Minors, Åkerstedt, Reilly, & Atkinson, 2001 , for example). In the present context, " mental performance " can be defi ned operationally as the outcome of a task, effort, or activity that engages the central nervous system (CNS).The combined result of these studies is that many of the factors that infl uence a person ' s ability to perform a task successfully are known. These factors include ambient conditions, food intake, practice effects, chronotype, and the nature of the performance task. Additional factors, which will be considered in more detail, include the effects of time of day (circadian rhythmicity)...