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).

Sleep Propensity

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

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.)

Chemical Countermeasures

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).

Napping

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.

Activity Breaks

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]).

Exercise

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.