From Code 7700: Fatigue. Fatigue refers to a physiological state in which there is a decreased capacity to perform cognitive tasks and an increased variability in performance as a function of time on task. Fatigue is also associated with tiredness, weakness, lack of energy, lethargy, depression, lack of motivation, and sleepiness. Sleep Inertia. Sleep inertia (also termed sleep drunkenness) refers to a period of impaired performance and reduced vigilance following awakening from the regular sleep episode or nap. This impairment may be severe, last from minutes to hours, and be accompanied by micro-sleep episodes. Window of Circadian Low (WOCL). Individuals living on a regular 24-hour routine with sleep at night have two periods of maximum sleepiness, also known as “WOCLs.” One occurs at night, roughly from 3 a.m. to 5 a.m., a time when physiological sleepiness is greatest and performance capabilities are lowest. The other is in the afternoon, roughly from 3 p.m. to 5 p.m. Sleep-Related Processes [AC 120-100, ¶7.] Sleep Regulation. The drive for sleep increases over time since the last sleep period and with any cumulative deficit in sleep relative to the average 8-hour day requirement. As a consequence, the sleep drive is at its lowest point in the morning, upon awakening, and as the day progresses, the drive to sleep increases and the ability to sustain attention and engage in cognitive activities decreases. Once sleep begins, this drive gradually decreases until awakening. Elevated Sleep Drive. For the average person, the daily upswing in alertness produced by the circadian system tends to offset the decrease in alertness produced by depletion of the sleep regulatory process. The result is roughly constant reaction time and lapses during the first 16 hours of the day 85. After about 16 hours of continuous wakefulness, most adults begin to notice reductions in the speed of performance and in alertness levels 87. However, a prior history of insufficient sleep quantity and quality can magnify the changes in behavior and alertness. Desynchronization. The timing of sleep and wakefulness of most humans, under natural conditions, is consistent with the circadian control of the sleep cycle and all other circadian-controlled rhythms. However, people working in a developed society override their internal biological clock and attempt to sleep at times that are not always consistent with the biological drive to sleep. For example, when individuals travel rapidly across time zones or work the night shift, the sleep/wake cycle is out of phase with the biological rhythms controlled by the circadian clock. This can adversely affect both alertness while awake and at work, and the ability to achieve restorative sleep. Sleep Inertia. This sleep-related process causes a temporary degradation in performance immediately after awakening. The degradation or loss of alertness is dependent on depth of sleep at the time of awakening. The degradation dissipates, after awakening, on a time scale ranging from minutes to a few hours. Sleep inertia causes a feeling of drowsiness or lethargy and can be measured as a noticeable change in reaction time and potential for lapses in attention. The duration and severity of sleep inertia is related to the depth of sleep at the time of awakening. It tends to be greater after short sleep periods of an hour or two, when the need for sleep is not fully satisfied, or after sleep when the person is carrying a large sleep debt from prior sleep restrictions 10. Fatigue Factors   Figure: Window of circadian low, from Duty/Rest Guidelines for Business Aviation, §1.0. [Duty/Rest Guidelines for Business Aviation, §1.0] 1.1 Sleep Sleep is a vital physiological need. Sleep is necessary to maintain alertness and performance, positive mood, and overall health and well-being. Each individual has a basic sleep requirement that sustains optimal levels of performance and physiological alertness during wakefuln