Cognitive performance degrades selectively under sleep loss. Thus, it appears that specific demands associated with given cognitive task influence the performance results under sleep loss. However, most of the studies that examine the effects of sleep deprivation on cognitive performance have focused on measures that are sensitive to sleepiness, such as simple reaction time, but have little to do with the true nature of cognitive demands in real work performance. Some other studies have reported that sleep loss does not lead to detrimental effects on higher cortical functioning, as measured with neuropsychological tests that have been mainly developed for detecting different disorders. But in these studies participants have been young and screened for not having any neurological disorders. In this presentation I will focus on human cognitive processes and some other critical factors for selecting cognitive measures to evaluate the effects of sleep loss on performance. The study of five days partial sleep loss and following two days with 8 hours sleep will be presented as an example. In this study, cognitive performance has been measured repeatedly in multitasking, the declarative and procedural memory tasks and the working memory tasks in each day of the experiment. The cognitive performance declined selectively in multitasking and procedural memory performance, while declarative and working memory performance were very little affected by sleep loss. In sum, the more systematic studies evaluating the effects of sleep loss on human cognitive processes are needed.