Cognitive and affective processes vary over the course of the 24 h day. Time of day
dependent changes in human cognition are modulated by an internal circadian timekeeping
system with a near-24 h period. The human circadian timekeeping system interacts with
sleep-wakefulness regulatory processes to modulate brain arousal, neurocognitive and
affective function. Brain arousal is regulated by ascending brain stem, basal forebrain
(BF) and hypothalamic arousal systems and inhibition or disruption of these systems
reduces brain arousal, impairs cognition, and promotes sleep. The internal circadian
timekeeping system modulates cognition and affective function by projections from the
master circadian clock, located in the hypothalamic suprachiasmatic nuclei (SCN), to
arousal and sleep systems and via clock gene oscillations in brain tissues. Understanding
the basic principles of circadian and wakefulness-sleep physiology can help to recognize
how the circadian system modulates human cognition and influences learning, memory
and emotion. Developmental changes in sleep and circadian processes and circadian
misalignment in circadian rhythm sleep disorders have important implications for learning,
memory and emotion. Overall, when wakefulness occurs at appropriate internal biological
times, circadian clockwork benefits human cognitive and emotion function throughout
the lifespan. Yet, when wakefulness occurs at inappropriate biological times because of
environmental pressures (e.g., early school start times, long work hours that include work
at night, shift work, jet lag) or because of circadian rhythm sleep disorders, the resulting
misalignment between circadian and wakefulness-sleep physiology leads to impaired
cognitive performance, learning, emotion, and safety.