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Influence of the circadian timekeeping system on cognitive function in humans

Kenneth P. Wright Jr
Sleep and Chronobiology Laboratory, Department of Integrative Physiology, University of Colorado at Boulder , USA

The circadian timekeeping system regulates near-24-hour variations in physiology and behavior. The most common markers of internal biological time in humans, representing the output of the SCN, are circadian rhythms of the pineal hormone melatonin and of core body temperature. Wakefulness and its associated functions are promoted during the biological day when melatonin levels are low and body temperature levels are high, whereas sleep and its associated functions are promoted during the biological night when melatonin levels are high and body temperature levels are low. Cognitive function is generally worst near the minimum phase of the core body temperature rhythm. It is impossible however, to completely separate the influence of circadian phase from that of wakefulness-sleep related processes on human physiology and behavior since these fundamental CNS processes interact. Wakefulness and sleep are in part regulated by the circadian timing system and in turn, wakefulness-sleep processes influence the circadian system. Changes in the phase relationship between internal circadian phase and wakefulness-sleep timing have important implications for cognitive function. Individual differences in cognitive function appear to be greatest near the temperature minimum, especially when homeostatic sleep drive is high. Whether individual differences in cognitive function are related to circadian and/or wakefulness-sleep processes is unknown.