This chapter discusses the three domains of cognition for which the effects of sleepiness are reasonably well understood: attention and vigilance, executive functioning, and learning and memory. It investigates the effects of sleepiness on distinct cognitive processes involved in the performance of cognitive tasks. Sleepiness due to sleep loss or circadian misalignment causes an increase of average reaction times in vigilance and reaction time tasks. Sleepiness has the potential to alter emotional states and bias the interpretation of the emotional context of a situation, and may thereby influence decision-making. Memory encoding, or the conversion of sensory input into a neural representation, appears to be adversely affected by lack of prior sleep. Sleepiness may affect these cognitive processes differentially, so that the effects of sleepiness depend on the ensemble of cognitive processes required to perform the task at hand.