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

3. What is fatigue? Section 1

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. Effects on various performance tasks

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

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. 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. A typical sleep period

14. The homeostatic process

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

17. The two components of alertness / fatigue

18. Combined during a normal day

19. Combined overnight

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. Tracking performance at different times of day