Circadian rhythms in physiology and behavior exist in all living organisms, from cells to humans. The most evident rhythms are the recurrent cycles of sleep and wake as well as changes in alertness and cognitive performance across the 24 h. Clearly, sleep pressure can exert a strong influence on cognitive performance, but the influence of circadian modulation of alertness and cognitive function is evident even when the pressure for sleep is high. Circadian rhythms also influence more complex cognitive tasks, such as selective attention and executive function, which are important for work performance and safety.The circadian timekeeping system also ensures that circadian rhythms are appropriately synchronized to the external physical environment and work and social schedules. Circadian misalignment is the basis for all circadian rhythm sleep disorders. These disorders are often associated with impairments of cognitive performance that can have adverse effects on school and work performance, overall quality of life, and safety.