The effect of fatigue on commercial motor vehicle (CMV) drivers is an important public safety issue. The National Transportation Safety Board has identified fatigue as a probable cause or a contributing factor in incidents and accidents across all modes of transportation. It is important to understand how fatigue affects performance and the implications of these effects for highway safety.1
Safe motor vehicle operation requires, among other things, the ability to stay awake and sustain maintenance of stable vigilance, situational awareness, and appropriately timed psychomotor and cognitive responses. However, these are the neurobehavioral functions affected most immediately and profoundly by work-hour fatigue and insufficient sleep. As noted in Chapter 1, fatigue refers to increasing performance variability, instability in behavioral alertness, and decreasing vigilance due to continued time on task without breaks (Lim et al., 2010; Neri et al., 2002) or insufficient sleep (Basner and Dinges, 2011; Lim and Dinges, 2008). The contribution of insufficient sleep to performance deficits in CMV driving is a reasonable concern given the scientific evidence that obtaining sufficient sleep is essential for optimal performance of tasks central to operating a motor vehicle safely (Banks and Dinges, 2007; Lim and Dinges, 2008, 2010).
1 This chapter deals with fatigue and not distraction, which is also a common reason for highway crashes. Fatigue and distraction can be interactive. For more information on distraction, see Thiffault (2011).
Population studies have found that Americans reduce their sleep time primarily in favor of work time, commute time, and leisure time, in that order of importance (Basner and Dinges, 2009; Basner et al., 2007, 2014). As noted in Chapter 1, insufficient sleep is currently defined as sleeping less than 7 hours per day (Ford et al., 2015; Watson et al., 2015a, 2015b). This number derives from two sources. The first is randomized controlled laboratory experiments that systematically varied the duration of sleep obtained by healthy adults every day for 1 to 2 weeks, from 3 to 9 hours, while measuring a range of neurobehavioral performance functions each day (Belenky et al., 2003; Van Dongen et al., 2003a). Cumulative deficits in psychomotor speed and vigilance lapses were observed when sleep was below 7 hours per day, in accordance with the current definition of insufficient sleep. Importantly, the rate of performance deterioration was inversely related to the daily dose of sleep (i.e., the less sleep obtained per day, the more rapidly these deficits increased). Moreover, dividing the sleep into two periods each day (e.g., nocturnal sleep and daytime nap) did not have adverse effects on performance until the total time for sleep each day was below 7 hours (Mollicone et al., 2007, 2008), except that deficits when there was a nighttime circadian pressure for sleep did reveal performance deficits from not having a continuous sleep time (Mollicone et al., 2010).
There is some evidence of a (partially determined) genetically based difference among individuals as to sleep need, resistance to sleep deprivation, and circadian phase preference for sleep (see, for example, Goel et al., 2010; He et al., 2009; Pellegrino et al., 2014). However, the linkage between differences in phenotypic sleep need and vulnerability to sleep loss are not yet well enough understood neurobiologically to permit reliable prediction of behavioral risk. At this point, moreover, it is not possible to make use of genetic information to set more individualized hours-of-service (HOS) regulations. Therefore, this report makes no further mention of this issue.
Chronic insufficient sleep due to sleeping less than 7 hours per day results in a “sleep debt” (Van Dongen et al., 2003b) that gradually accrues over time, resulting in a greater tendency or propensity to fall asleep unintentionally (McKnight-Eily et al., 2009; Punjabi et al., 2003), and is associated with more reports of greater drowsiness when driving (Abe et al., 2012; Scott et al., 2007). Sleep debt can be reduced only by extending sleep time for recovery. Stimulants, such as caffeine, nicotine, and other drugs used to promote wakefulness, can increase arousal and transiently
improve performance in fatigued individuals, but their effects are limited, and those who take them do not recover their sleep debt because there are no chemical substitutes for sleep (Bonnet, 2005; Spaeth et al., 2014).
Individuals who obtain less than 7 hours of sleep per day because of work and other activities typically sleep longer on nonworkdays to recover the sleep debt (Basner et al., 2014). This appears to be the case for CMV drivers as well. In the past 10 years, three naturalistic studies of CMV drivers have used wrist actigraphy devices to record objectively drivers’ sleep times and sleep durations per 24 hours, on duty and nonduty days (Dinges et al., 2005b; Hanowski et al., 2007; Van Dongen and Mollicone, 2014). These studies revealed that the amount of sleep obtained by the drivers on workdays averaged 5.0 to 6.2 hours per 24 hours, while their sleep on off-duty days averaged 6.5 to 8.9 hours per day. In all three studies, the differences in sleep time on work- versus nonworkdays were statistically significant, and in all three, drivers’ workday sleep durations averaged below 6.3 hours per day, an amount of daily sleep time considered insufficient for health (Watson et al., 2015a, 2015b). Thus, these studies suggest that on average, CMV drivers accumulate some degree of sleep debt due to insufficient sleep on workdays, and that they attempt to reduce that debt by sleeping 1-2 hours longer on off-duty days—a behavioral pattern of many adult Americans (Basner et al., 2014). Laboratory evidence indicates that a compensatory increase in sleep duration (i.e., being able to sustain sleep for a longer period of time) following 5 nights of sleep restriction occurs on 1 or more recovery nights in response to insufficient sleep (Banks et al., 2010). It is not known whether repeatedly cycling between 5-7 days of reduced sleep time per day and 1-2 days of extended (recovery) sleep has consequences for health and safety. Even if this pattern makes it possible for drivers to recover from workweek sleep debt, inadequate sleep during a workweek carries risks, as described below.
Fatigue from Inadequate Sleep
Studies have repeatedly shown that fatigue most often occurs as a result of the physiological consequences of inadequate sleep, prolonged wakefulness, and being awake at a circadian time when the brain is programmed to sleep. (The circadian nadir in alertness typically occurs between 11 PM and 7 AM, but can vary among individuals by a few hours, depending on habitual sleep timing.) These factors can co-occur to amplify the adverse effects of fatigue on performance and behavior.
Inadequate sleep also can result from lack of treatment of common medical conditions such as insomnia and sleep apnea, both of which have a high prevalence in the general population, although their frequencies
in CMV drivers are not well established. Chronic insomnia is defined as the subjective perception of difficulty with sleep initiation, duration, consolidation, or quality that occurs despite adequate opportunity for sleep, and that results in some form of daytime impairment (Schutte-Rodin et al., 2008). Insomnia has been linked to lost work time and workplace accidents (Kucharczyk et al., 2012; Leger and Baron, 2010). If untreated, sleep apnea can result in excessive daytime sleepiness and risks to safety, especially when one is driving.
Sleep loss has a wide range of adverse effects on cognitive domains and neurobehavioral functions. These effects include (1) unstable attention, evident in both errors of omission (i.e., failure to respond in a timely manner to a stimulus) and errors of commission (i.e., responses when no stimulus or the wrong stimulus is present), as well as increased decrements in vigilance; (2) slowing of cognitive and psychomotor response times; (3) decline of both short-term and working memory performance; (4) reduced learning (acquisition) of cognitive tasks; (5) deterioration of performance in tasks requiring divergent thinking; (6) perseveration with ineffective solutions; (7) performance deterioration as task duration increases; and (8) growing neglect of activities judged to be nonessential (Goel et al., 2009).
Sleep Dose-Response Studies
Results of controlled laboratory experiments indicate that the effects of chronic sleep restriction are especially apparent when sleep is restricted to less than 7 hours a night (Belenky et al., 2003; Dinges et al., 1997; Van Dongen et al., 2003a). In these studies, performance deficits increased steadily across consecutive days of sleep restricted to less than 7 hours, and the less sleep was obtained per night, the more rapidly the performance deficits increased across days of sleep restriction. Within 5 to 6 days of sleep restricted to less than 7 hours, decrements in behavioral alertness increased to levels equivalent to having had no sleep at all for 24 to 48 hours (Van Dongen et al., 2003a, 2003b).
In studies of sleep duration relative to daytime sleep propensity (i.e., the physiological tendency to fall asleep) and drowsy driving, adults reporting sleep durations of 6.75 to 7.5 hours and of less than 6.75 hours had a 27-percent and 73-percent increase, respectively, in the risk of sleep onset during the sleep propensity test compared with adults reporting more than 7.5 hours of sleep (McKnight-Eily et al., 2009; Punjabi et al., 2003). Studies also have shown that motor vehicle crash risk increases
when self-reported sleep duration is less than 6 hours per day (Abe et al., 2012; Scott et al., 2007). A recent cross-sectional survey of drivers found an association between self-reported sleep duration of less than 7 hours per day and at least one self-reported incident of falling asleep while driving during the prior year.
Self-Reported Measures of Sleepiness and Fatigue
Dose-response studies on the adverse effects of sleep restriction on attention and performance have shown that self-reported sleepiness or fatigue does not continue to increase with chronic sleep restriction, but rather achieves a maximum at levels that do not reflect actual performance risks. Therefore self-reported sleepiness and fatigue may not reliably reflect increasing performance risks.
Sleep inertia refers to a performance deficit that occurs upon awakening from sleep and that involves grogginess and a tendency to fall back asleep (Dinges, 1990). Although extended wakefulness without sleep can increase performance lapses, and sleep can reduce these effects, performance can be degraded by sleep inertia during the period immediately after awakening from sleep (for up to 2 hours, depending on the severity of prior sleep deprivation) (Dinges, 1990; Jewett et al., 1999). The more severe cognitive deficits of sleep inertia can be blocked by ingesting caffeine (Hayashi et al., 2003; Van Dongen et al., 2001).
Alcohol as a Contributing Factor
The fact that alcohol can impact the degree to which insufficient sleep affects a driver is becoming increasingly better understood. Alcohol appears to heighten the degree to which insufficient sleep impairs performance. (For further information, see Akerstedt et al., 2008.)
Fatigue management in operational environments in general and in transportation modalities in particular is a major priority in many parts of the world, and the focus of a growing amount of human factors research (Abe et al., 2015). Countermeasures for fatigue are a special area of focus in fatigue management.2
The stimulant most commonly used as a fatigue countermeasure by many people, including CMV drivers, is caffeine, which acts pharmacologically to enhance alertness. Research has shown that caffeine has alerting effects and increases performance levels in the short term, especially in those who do not consume high doses on a regular basis (Nehlig, 1999; Neri et al., 1995). Caffeine affects the nervous system within 15-20 minutes, and its alerting effects can last for 4 to 5 hours, depending on the biological rate at which an individual clears it. However, research on the extent to which caffeine can maintain performance as sleep deprivation continues over days has revealed that caffeine can be ineffective for maintaining alert levels of performance as time awake extends past 16 hours and in individuals with high sleep debt (Spaeth et al., 2014).
Although CMV drivers may consume caffeinated foods and beverages, as well as ingest nicotine, then, it is important to reiterate that there is no biological substitute for sleep (Bonnet et al., 2005). The only way to recover effectively from sleep loss is to obtain adequate recovery sleep through a prolonged daily sleep period and the strategic use of naps (see below). Extensive evidence shows that sleep is the only reliable, natural, effective countermeasure for mitigating the neurobehavioral deficits due to sleep loss (Banks et al., 2010; Dinges et al., 1987; Mollicone et al., 2007, 2008; Rogers et al., 2003; Van Dongen et al., 2001).
A great deal of research conducted in both laboratory and operational settings has found that naps are an effective way to restore alertness and counter sleepiness when the time available for sleep is limited (see, e.g., Bonnet, 1991; Brooks and Lack, 2006; Costa, 1997; Driskel and Mullen, 2005; Lavie, 1986; Rosekind et al., 1994; Schweitzer et al., 1992; Tilley et al., 1982). Naps taken both prior to (i.e., “prophylactic” naps) and during (i.e., “power” naps) work periods have been found to improve alertness and performance relative to not napping (Bonnet, 1991; Dinges et al., 1987; Schweitzer et al., 1992). Moreover, a meta-analysis of 12 studies found that naps led to performance improvements that were directly proportional to nap duration (Driskell and Mullen, 2005). Other studies also have found a nap dose-response benefit (Bonnet, 1991; Brooks and Lack, 2006).
To maximize the beneficial effects of naps on performance and physiological alertness, aspects of nap timing must be considered. Daytime naps are typically difficult to initiate and maintain because this is the time at which the circadian clock is programming daytime alertness (Costa, 1997; Lavie, 1986; Tilley et al., 1982). During the afternoon “siesta” period,
however, it is typically easier to initiate sleep in preparation for an upcoming night shift. Naps taken during the nighttime when the circadian clock is programming sleepiness are easier to maintain and show the most beneficial effects (Dinges, 1986). For example, a 1-hour nap at 4:30 AM was found to be more beneficial for next-day performance than a 1-hour nap at 9 PM (Gillberg, 1984). It is important to keep in mind that when awakening from naps taken during the circadian low point, individuals may experience a significant period of sleep inertia, or a feeling of grogginess, upon awakening, accompanied by decrements in performance.
People generally do not think of an activity break as an effective way to combat fatigue. However, research has shown that short breaks, especially when they include some physical activity, increase alertness by reducing the monotony of a task. The beneficial effects of breaks are due in part to postural changes that occur when one gets up and walks around (Caldwell et al., 2003; Dijkman et al., 1997; Heslegrave and Angus, 1985; Matsumoto et al., 2002). Studies have shown that short-term physiological and subjective alertness benefits lasting up to 15-30 minutes can be associated with activity breaks. However, performance on a test that is very sensitive to sleep loss (i.e., a psychomotor vigilance test) indicated no significant benefit of breaks (Neri et al., 2002). The beneficial subjective and physiological effects of the breaks were most pronounced around the low point in body temperature (i.e., the window of circadian low [Neri et al., 2002]).
As suggested above, periods of light-to-moderate exercise have been shown to increase physiological arousal and help promote alertness (Buxton et al., 2003; Eastman et al., 1995; Horne and Foster, 1995; LeDuc et al., 2000; Stepanski and Wyatt, 2003). The beneficial effects of moderate exercise on subjective sleepiness have been shown to last up to 30 minutes (LeDuc et al., 2000), but improvements in objective performance have not been observed.
The extent to which CMV drivers utilize countermeasures for fatigue during their routine work schedules is unknown. The few studies that have monitored their sleep times objectively have found a considerable difference in their sleep on workdays and on days off duty. On workdays,
they appear to obtain on average 6-6.5 hours per day, at the low end of what is considered sufficient sleep to maintain daily alertness and health. The additional sleep they obtain on days off duty (7-8.5 hours) suggests they are compensating for a cumulative sleep debt from the prior work week. The extent to which CMV drivers use naps, caffeine, nicotine, rest breaks, and exercise to mitigate their fatigue when working is unknown. Also unknown is whether their sleeping arrangements during work periods promote healthy physiological sleep continuity and duration, which are essential for optimizing waking alertness. Research is needed that can provide realistic estimates of the extent to which drivers are utilizing the above countermeasures, and the extent to which their use of those countermeasure facilitates alertness and performance.