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?
Section 1
What is fatigue?
4
Definition of fatigue
• The impairment of mental capability associated with the pattern of work and rest
• Not to be confused with sleepiness
Section 2
How can we measure
fatigue?
6
Effects on various performance tasks
performance
(baseline =1)
1.40
1.35
1.30
1.25
Continuous memory recall (RT)
Visual vigilance (errors)
Simple monitoring (RT)
Complex monitoring (RT)
Tracking (RMS error)
1.20
1.15
1.10
1.05
1.00
0.95
0.90
1715
1900
2045
2230 0015
time of day
0200
0345
0530
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
Section 3
How does fatigue arise?
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
performance
(%)
12
temp (o C)
37.6
8
37.4
4
37.2
0
37
-4
36.8
-8
-12
Sleep
Performance score
36.6
Temperature
-16
36.4
0900 1200 1500 1800 2100 2400 0300 0600 0900
time (h)
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
The homeostatic process
14
Sleep
propensity
Sleep
0
4
8
12
16
20
24
4
8
12
16
20
24
time of day
Sleep
propensity
Sleep
24
4
8
12
16
20
24
4
time of day
8
12
16
20
24
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)
15
16
The relationship with fatigue
the circadian
mechanism
the homeostatic
mechanism
alertness /
fatigue
17
The two components of alertness / fatigue
60
54
Alertness
Alertness
48
42
36
30
24
18
12
6
0
0
2
4 6
8 10 12 14 16 18 20 22 24
Time of day (h)
0
2
4
6
8
10
12
Time since sleep (h)
14
16
18
18
Combined during a normal day
Individual components
Combined value
Alertness
Alertness
60
70
50
60
50
40
40
30
30
20
10
Time since sleep component
20
Time of day component
10
0
0
6
8
10
12
14
16
Time of day (h)
18
20
22
6
8
10
12
14
16
Time of day (h)
18
20
22
19
Combined overnight
Individual components
Combined value
Alertness
Alertness
60
80
70
50
60
40
50
30
40
30
20
10
Time since sleep component
20
Time of day component
10
0
18
0
20
22
24
2
4
Time of day (h)
6
8
10
18
20
22
24
2
4
Time of day (h)
6
8
10
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)
Tracking performance at different times of day
T r a c k i n g p e r fo r m a n c e
(R M S e r ro r )
3 00
2 50
2 00
1 50
1 00
50
0
2 1 3 0 -2 2 4 5 2 3 0 0 -0 0 1 5 0 0 3 0 -0 1 4 5
0 2 0 0 - 0 3 1 5 0 3 3 0 -0 4 4 5 0 5 0 0 -0 6 1 5 0 6 3 0 -0 7 4 5
T im e o f d a y
21