There is rapidly accumulating evidence of a close relationship between sleep loss and cognition. Neuropsychologists need to become aware of this body of knowledge as the effects of sleep loss on brain functions are significant. The current study (a) outlines the extent to which insufficient sleep affects performance on cognitive tasks in otherwise healthy people, (b) discusses the relationship between sleep and neurocognitive disorders, and (c) highlights key issues that merit consideration for neuropsychologists. This review shows that sleep loss has a measurable impact on performance through decreases in cognitive functions and effects on biological pathways that support cognitive performance. Sleep loss reliably produces reductions in speed of processing and attention. Higher order cognitive functions are affected to a lesser extent, and there is sparing on tasks of crystallized abilities. Deficits worsen with increasing time awake, but may be overturned after normal sleep is resumed. The review also shows that sleep disorders are a major feature of neuropsychological conditions contributing to the pattern of cognitive impairment. Overall, neuropsychologists must be alert to sleep problems in their clients, so that sleep interventions, or referrals, are put in place in the rehabilitation plan of individuals with cognitive dysfunctions. Recommendations also include routine screening of sleep as part of cognitive assessment.