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Sleep and Fatigue

Here is a definition of fatigue:

Fatigue is a suboptimal psychophysiological condition caused by exertion. The degree and dimensional character of the condition depends on the form, dynamics and context of exertion. The context of exertion is described by the value and meaning of performance to the individual; rest and sleep history; circadian effects; psychosocial factors spanning work and home life; individual traits; diet; health, fitness and other individual states; and environmental conditions. The fatigue condition results in changes in strategies or resource use such that original levels of mental processing or physical activity are maintained or reduced (ICAO, 2016).

Hockey (2013) suggests that ‘fatigue’ has been ‘hijacked by the sleep scientists’ with important issues around what he refers to as psychological fatigue being overlooked. To further complicate matters, it is not at all clear whether fatigue is an actual condition or simply the symptoms associated with some underlying state. For these reasons, in the rest of this chapter, I want to distinguish between physiological ‘fatigue’ arising from sleep deprivation or disruption and psychological ‘fatigue’, which is a longer-term problem resulting from life’s challenges. The two aspects do, in fact, interact but warrant separate treatment.

Physiological fatigue, quite simply, is a failure to meet our sleep requirements and results in a state of sleep deprivation. The implications of sleep loss in terms of lost productivity and increased health care costs have resulted in an explosion of interest in, and research into, sleep but, in their review of the proposed changes to the Australian flight time limitation rules, the authors comment that ‘fatigue science is very rarely definitive or conclusive’ (Civil Aviation Safety Authority (CASA), 2018). The issue of fatigue management can be contentious, even political, in the aviation industry, but the fact remains that the topic is still poorly understood. Most national aviation legal frameworks require operators to implement ‘legal’ rosters but the regulations also place a responsibility on the individual to report for duty adequately rested and prepared. Reconciling these two positions is difficult.

What Is Sleep?

Sleep is, quite simply, a state of unconsciousness. The purpose of sleep seems to be to allow the body to replenish energy stores, repair the wear and tear endured during the day and to organise memory. While we sleep the brain exhibits four distinct types of activity.

  • • Stage 1, light sleep, involves the brain becoming less responsive to environmental stimuli.
  • • Stage 2 occurs throughout sleep and represents almost half of the total sleep time.
  • • Stage 3 is distinguishable by the amount of delta wave activity observable.
  • • Stage 4 is known as rapid eye movement (REM) sleep and represents about a quarter of the sleep period.

The sleep process is dynamic, and the brain will move through the various stages over cycles lasting approximately 90 minutes. The stages may vary in duration. For example, the first REM episode may last just 10 minutes while the final episode may last an hour. For most people, between 6 and 9 hours of sleep per night is normal although 8 hours of sleep is considered necessary to be fully restorative. It seems that no single ‘sleep centre’ exists in the brain. Instead, a series of sleep circuits take inputs from the brainstem and the hypothalamus. The sleep cycle does appear to be synchronised with the body’s circadian rhythm with periods of wakefulness associated with a rise in the body core temperature while sleepiness coincides with a fall in core temperature. Two physiological processes regulate the need for sleep. First, during the hours of wakefulness, adenosine, a by-product of energy consumption, builds up in the body, and this increase triggers the need for sleep so that energy reserves can be replenished. Second, exposure to light produces melatonin. During the hours of darkness, melatonin production is inhibited and that synchronises the sleep cycle with the period of darkness. Humans are diurnal animals, after all.

The fundamental regulation of sleep - known as the homeostatic model - is disrupted by the very nature of work in aviation. Different types of operation - air taxi, helicopter, regional, low-cost, long-haul, cargo - all have different characteristics but all share some of the following issues:

  • • The work itself can be fatiguing because of the exposure to environmental stressors. It has been suggested that older pilots, especially those who smoke, could be in a constant state of marginal hypoxia during the cruise because of the relative cabin altitude.
  • • The work schedule usually has variable start times to the duty period and, especially for long-haul flying, could be classified as a form of quasi-shift work. Both conditions have been linked to health issues.
  • • Passage through time zones and the subsequent disruption of sleep patterns is a source of fatigue.
  • • Demands for productivity and disrupted schedules can affect recovery time between tasks.

Sleep deprivation studies have shown that after 24 hours of wakefulness, the brain’s metabolic activity is reduced by 6%, with reductions of up to 11% in specific brain centres. Higher-order cognitive tasks are affected earlier than other brain activities. Where tasks require both speed and accuracy, experiments have shown that speed of response decreases before accuracy is impaired. What is more significant is that the performance can be affected by apparently small reductions in sleep. Subjects restricted to 7 hours of sleep a night for 1 week showed degraded performance in tests of response time and in mathematical problem-solving. Tests in driving simulators showed a reduced ability to focus on several tasks at once, and accidents in simulations increased as sleep duration was reduced from 7 to 5 and then 3 hours per night. Sleep deprivation seems to have the greatest impact on motor control whereas psychological fatigue seems to affect the brain’s executive control functions (Hockey). Physiological fatigue can be considered to be acute or chronic. Acute fatigue is a sudden onset condition and is mitigated by sleep. Chronic fatigue is an underlying, persistent state not necessarily mitigated through access to sleep.

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