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

3. What is fatigue? Section 1

4. 4 Definition of fatigue The impairment of mental capability associated with the pattern of work and rest Not to be confused with sleepiness

5. How can we measure fatigue? Section 2

6. 6 Effects on various performance tasks

7. 7 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

8. How does fatigue arise? Section 3

9. 9 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

10. 10 Diurnal variation in temperature From Klein et al, 1976

11. 11 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)

12. 12 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

13. 13 A typical sleep period

14. 14 The homeostatic process

15. 15 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)

16. 16 The relationship with fatigue the circadian mechanism the homeostatic mechanism alertness / fatigue

17. 17 The two components of alertness / fatigue

18. 18 Combined during a normal day

19. 19 Combined overnight

20. 20 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)

21. 21 Tracking performance at different times of day