Lack of sleep may have severe consequences on health and safety. Understanding the effects of sleep deprivation on human behavior and brain functions has important relevance in our society in which the lack of sleep is increasing. The aim of this dissertation was to elucidate the impact of sleep loss on cognition and motivation and to highlight in this context the crucial importance of individual differences in the vulnerability to sleep loss. Four experiments are presented in this work. Overall the findings suggest that sleep deprivation does not affect basic automatic functions and instincts in presence of relevant stimuli. Moreover, differences between individuals in vulnerability to sleep deprivation are crucial and have to be considered in sleep deprivation research.