Sleep and Performance
The effects of sleepiness, sleep loss, and fatigue have been the focus of literally hundreds of studies dating back to a study conducted by Patrick and Gilbert (1896) at the University of Iowa. These scientists studied the effects of keeping a group of subjects awake for over 90 hours. Using performance tests measuring reaction time, motor speed, and memory they demonstrated the deleterious effects of sustained wakefulness.
The "modern" study of sleep and performance began with the work of Williams (1959) who demonstrated that there was a progressive increase in reaction time across days of sleep deprivation. These findings wer evident regardless of the nature of the reaction time task, the duration of the task, and whether the person received feedback on how they were doing.
A little over 100 years after Patrick and Gilbert (1896), Heslegrave and Rhodes (1997) reported that air traffic controllers perceived some degree of performance impairment the end of an 8-hour day and evening shift, but significantly greater performance impairment at the end of an 8-hour midnight shift. In fact, the degree of performance impairment at the end of an 8-hour midnight shift was similar to that of the end of a 12-hour day or evening shift. In terms of sleep, when air traffic controllers (ATCs) worked the midnight shift, they reported only about 5 hours of sleep on a daily basis and only about 6-6.5 hours on day shifts. In terms of this aging population, controllers reported more difficulty with shift-work beginning between 35-39 years of age.
In addition to perceived performance decrements, performance changes across various shifts were also demonstrated. For example, using timed reaction time, reasoning, and spatial relations tasks, ATCs performance began to deteriorate 5-10% on the second midnight shift and by the fourth midnight shift a reduction in performance of 10-18% from baseline was observed. For the EDDMM (evening-day-day-midnight-midnight) shift, significant performance deterioration did not occur until the midnight shifts with a 6-12% reduction in reasoning,. Spatial orientation, and pattern recognition. For the EEDDMM shift, performance impairment of 5-15% was evident during the second day shift and during the midnight shift.
Luna, French, and Mitcha (1997) also reported that ATCs on the night-shift of a forward rapid rotation shift schedule appeared to be falling asleep and reported increasing confusion and fatigue.
Many railroad employees do not have a predictable work schedule like the ones described above. One study by Hildebrandt, Rohmert, and Rutenfranz (1974) found that locomotive engineers were more likely to have difficulty using their safety alerters during the night. A laboratory study by Thomas, Raslear, and Kuehn (1996) also reported some indications of performance decrements in locomotive engineers. Never the less, the implication from these studies is that employees with an erratic work schedule (i.e. one that doesn’t correspond to the so-called typical 8-hour day) may be more likely to experience performance deterioration and decrements.
Summary of Effects of Fatigue on Performance
A number of key findings have been summarized by various authors over the past few years relating to fatigue and rest in the occupational setting involving continuous operations. The following list was generated from a review of several articles published by Rosekind (1995, 1996) and Dinges (1991, 1995). They are listed in the form of points to aid in grasping the significance of the findings. Those interested in a more detailed review of the findings should consult the original sources.
- Sleep deprivation results in cognitive performance deficits
- Disruption of circadian rhythms leads to a decrease in performance
- Human beings are not very good at estimating their current level of alertness
- Repeated disruption of sleep schedules can lead to decreased performance
- Sleep inertia can lead to performance decrements
- Inability to get regular sleep may lead to disruption of the circadian rhythm
- Short naps have been found to restore an individual’s capacity for performance under certain conditions
- Time off alone may not guarantee a rested workforce. Education, planning, and predictability are needed to maximize utilization of work/rest schedules
- With repeated loss of sleep, a sleep debt builds up over time
- With increased sleep loss and increased sleepiness a person may become vulnerable to performance problems
- Quality of sleep is an important factor. Poor quality sleep can leave a person feeling fatigued and non-restored
- There can be a discrepancy between how people are feeling and how sleepy they are physiologically
- Scientific evidence suggests that being on an altered shift schedule, like nights, does not lead to an altered internal circadian pattern
- Shift workers that go back and forth between shifts experience more difficulties between the circadian rhythms and sleep times
- Moving a shift schedule forward involves easier physiological adaptations
The following conclusions regarding the effects of sleep loss and sleepiness were abstracted from several articles written by David Dinges of the University of Pennsylvania (Dinges, 1991, 1995). They cover several aspects of research, which are relevant to transportation operations.
- Fatigue affects performance
- Fatigue erodes performance such as vigilance and sustained attention
- Four main processes affect performance decrements: circadian phase, acute sleep loss, cumulative sleep loss, and sleep inertia
- Fatigue can cause lapses or micro sleeps
- Night work, which interferes with the circadian rhythm, can cause lapses to increase 4 to 10 fold
- The longer one is awake beyond 14-16 hours, the greater the occurrence of lapses
- Chronic under-sleeping creates a cumulative sleep debt, which can produce a cumulative increase in lapses on vigilance tasks
- Sleep inertia creates lapses
- Fatigue produces a variety of performance decrements:
- Performance variability
- Slowed physical and mental reaction time
- Increase in number of work related errors
- Increased tendency to persistently repeat behaviors
- Increase in false responding
- Increases memory errors
- Decreased vigilance
- Reduced motivation and laxity
- Magnitude of fatigue effects vary by individual
This limited review should serve to alert readers that the "science of fatigue and alertness" is still in its infancy. Those looking for definitive "proof" of a particular point of view are likely to be disappointed. The results of investigations to this point can only be used to generate basic guidelines to follow in developing fatigue countermeasures.