Phenotyping of neurobehavioral vulnerability to circadian phase during sleep loss.

The two-process model of sleep-wake regulation posits a neurobiological drive for sleep that varies homeostatically (increasing as a saturating exponential during wakefulness and decreasing in a like manner during sleep) and a circadian process that neurobiologically modulates both the homeostatic drive for sleep and waking alertness and performance. Endogenous circadian rhythms in neurobehavioral functions, including physiological alertness and cognitive performance, have been demonstrated using laboratory protocols that reveal the interaction of the biological clock with the sleep homeostatic drive. Acute total sleep deprivation and chronic sleep restriction increase homeostatic sleep drive and degrade waking neurobehavioral functions as reflected in sleepiness, attention, cognitive speed, and memory. Notably, there is a high degree of stability in neurobehavioral responses to sleep loss, suggesting that these individual differences are trait-like and phenotypic and are not explained by subjects' baseline functioning or a number of other potential predictors. The Psychomotor Vigilance Test is an important tool for phenotyping as it is sensitive to acute total sleep deprivation and chronic sleep restriction, is affected by the circadian and sleep homeostatic drives, shows large intersubject variability in the response to sleep loss, and tracks recovery from sleep restriction. Careful phenotyping is critical to accurately predict human performance (and individual differences) in situations in which the circadian and sleep homeostatic systems are perturbed such as acute total sleep loss, chronic sleep restriction, intermittent sleep loss, shift work, and jet lag.