Fatigue theory

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Fatigue theory

• Mick Spencer
• A presentation at the Royal Aeronautical Society
  seminar on Working Hours and Fatigue in Aviation
  Maintenance, RAF Bentley Priory
• 24.10.2002

2
Contents

• 1 What is fatigue?
• 2 How do we measure it?
• 3 How does it arise?

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What is fatigue?

• Section 1

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Definition of fatigue

• The impairment of mental capability associated
  with the pattern of work and rest
• Not to be confused with sleepiness

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How can we measure fatigue?

• Section 2

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Effects on various performance tasks
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Some measures of fatigue

• Performance tasks vigilance, sustained
  attention, mathematical logical reasoning,
  visuo-motor coordination, etc.
• Subjective assessments of fatigue of task
  performance
• The electrical activity of the brain e.g.
  changes in the theta, alpha, gamma bands

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How does fatigue arise?

• Section 3

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Basic mechanisms

• Two principal mechanisms that determine levels of
  fatigue are
• the circadian pacemaker or body clock fatigue
  levels tend to be higher at some times of day
  than others
• sleep (homeostatic mechanism) fatigue levels
  depend on the time since sleep and the duration
  of the previous sleep period

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Diurnal variation in temperature
From Klein et al, 1976
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The circadian mechanism

• The brain contains a circadian pacemaker that
  serves to synchronize physiological
  psychological functions with respect to time of
  day
• Under normal conditions it is entrained to the
  24-hour day
• Its intrinsic period is slightly greater than 24
  hours
• It responds to various time cues (zeitgeber), the
  most important of which is light
• Levels of fatigue tend to be highest close to the
  low point of the temperature rhythm (early to
  mid-morning)

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Stages of sleep

• Stage 0 - awake
• Stage 1 - drowsy or light sleep
• Stage 2 - normal sleep
• Stage 3 - slow-wave or deep sleep
• Stage 4 - slow-wave or deep sleep
• Rapid-eye movement (REM) sleep

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A typical sleep period
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The homeostatic process
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The interaction between the circadian and
homeostatic mechanisms

• There is a strong link from the circadian to the
  homeostatic mechanism
• the circadian rhythm influences the timing of
  sleep - particularly wake-up time
• as a result, individuals whose circadian rhythms
  are disturbed, are liable to experience sleep
  disruption
• There is a weak link from the homeostatic to the
  circadian mechanism
• the timing of sleep may act as a time cue
  (perhaps through its effect on light)

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The relationship with fatigue
the circadian mechanism
the homeostatic mechanism
alertness / fatigue
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The two components of alertness / fatigue
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Combined during a normal day
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Combined overnight
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Task-related factors

• The level of fatigue can also depend on the
  intensity of the work being undertaken (workload)
• The effect of time-on-task is most acute during
  tasks requiring sustained attention
• This effect can be overcome by short breaks in
  activity (approx. 15 minutes)

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Tracking performance at different times of day