Sleep and cognition are temporally regulated by a homeostatic process generating pressure for sleep as a function of sleep/wake history, and a circadian process generating pressure for wakefulness as a function of time of day. Under normal nocturnal sleep conditions, these two processes are aligned in such a manner as to provide optimal daytime performance and consolidated nighttime sleep. Under conditions of sleep deprivation, shift work or transmeridian travel, the two processes are misaligned, resulting in fatigue and cognitive deficits. Mathematical models of fatigue and performance have been developed to predict these cognitive deficits. Recent studies showing long-term effects on performance of chronic sleep restriction suggest that the homeostatic process undergoes gradual changes that are slow to recover. New developments in mathematical modeling of performance are focused on capturing these gradual changes and their effects on fatigue. Accident risk increases as a function of fatigue severity as well as the duration of exposure to fatigue. Work schedule and accident rate information from an operational setting can thus be used to calibrate a mathematical model of fatigue and performance to predict accident risk. This provides a fatigue risk management tool that helps to direct mitigation resources to where they would have the greatest mitigating effect.

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