Sleep Loss and Workload: Cognitive Effects and Physiology
Honn, Kimberly Ann
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This dissertation aims to investigate some of the complex interactions between sleep loss, workload, cognition, and physiology. In order to examine the effects of workload on fatigue (operationally defined as a state of reduced alertness) and performance, commercial airline pilots flew two duty schedules in a high-fidelity, moving-base simulator. The duty schedules were of equal duty duration and start time, but had differing levels of workload related to the number of flight segments flown. The higher workload condition, with more flight segments, had higher levels of fatigue, as shown through subjective measurements and through impairment in vigilant attention. To further investigate these effects, we conducted three laboratory studies. First, we present the validation of a tool used to measure impairment in vigilant attention related to sleep loss and fatigue. Second, we compared this performance impairment with measures of heart rate variability (HRV), reflecting autonomic nervous system activity. Performance impairments between and within test sessions during sleep deprivation were not reflected by changes in HRV. Last, we present data pertaining to the effect of sleep loss on additional components of cognition. Subjects took repeated administrations of six variants of a Digit Symbol Substitution Task (DSST), each designed to omit cognitive components potentially involved in the task, including visual search, spatial memory, paired associate learning, and motor memory. During 38 hours of sleep deprivation, all task variants showed performance impairment, regardless of which cognitive components were omitted. Together, these studies provided evidence concerning the underlying mechanisms of the sleep deprivation-induced impairment. Because the HRV measures did not vary in tandem with the performance impairment, the underlying mechanism may not be global in nature, that is, not general cognitive slowing or wake-state instability. While not all components of the DSST were controlled for, the presence of the sleep deprivation-induced performance impairment on all variants makes it less plausible that sleep deprivation causes deficits to specific cognitive functions. Instead, the physiological mechanism by which sleep deprivation induces cognitive impairments may be local or task-specific, such as the use-dependent degradation of the specific neural processes subserving the task.