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 field conditions. Mental performance is negatively affected by sleep loss; although short naps are beneficial, 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 field-based observations are relevant to the issue of learning in schoolchildren and lead to suggestions for improving their performance.