Sleepiness, also referred to as drowsiness, is defined in this report as the need to fall asleep, a process that is the result of both the circadian rhythm and the need to sleep (see below). Sleep can be irresistible; recognition is emerging that neurobiologically based sleepiness contributes to human error in a variety of settings, and driving is no exception (Åkerstedt, 1995a, 1995b; Dinges, 1995; Horne, 1988; Sharpley, 1996; Martikainen, 1992). In the more recent surveys and reporting of noncommercial crashes, investigators have begun to collect and analyze data for instances in which the driver may have fallen asleep. The terms "fatigue" and "inattention" are sometimes used interchangeably with sleepiness; however, these terms have individual meanings (Brown, 1994). Strictly speaking, fatigue is the consequence of physical labor or a prolonged experience and is defined as a disinclination to continue the task at hand. In regard to driving, a psychologically based conflict occurs between the disinclination to drive and the need to drive. One result can be a progressive withdrawal of attention to the tasks required for safe driving. Inattention can result from fatigue, but the crash literature also identifies preoccupation, distractions inside the vehicle, and other behaviors as inattention (Treat et al., 1979). The driving literature before 1985 made little mention of sleepiness and instead focused on the prevention of inattention and fatigue; traffic crash forms did not have a category for reporting sleepiness as a crash cause. Certainly, sleepiness can contribute to fatigue and inattention, and given the lack of objective tests or uniform reporting requirements to distinguish these different crash causes, misclassification and inconsistencies in the primary data and the literature can be expected. Some, but not all, recent studies and reviews make an explicit assumption that given the uncertainty in crash reports, all crashes in the fatigue and inattention categories should be attributed to sleepiness. The panel suspects that sleepiness-related crashes are still very often reported in the categories of fatigue and inattention, and it reached consensus that sleepiness is an underrecognized feature of noncommercial automobile crashes. The panel concluded that the data on fatigue and inattention provide less support for defining risk factors and high-risk groups than the data on sleepiness or drowsiness. In addition, sleepiness is identifiable, predictable, and preventable. THE SLEEP-WAKE CYCLE A body of literature exists on the mechanisms of human sleep and sleepiness that affect driving risks. The sleep-wake cycle is governed by both homeostatic and circadian factors. Homeostasis relates to the neurobiological need to sleep; the longer the period of wakefulness, the more pressure builds for sleep and the more difficult it is to resist (Dinges, 1995). The circadian pacemaker is an internal body clock that completes a cycle approximately every 24 hours. Homeostatic factors govern circadian factors to regulate the timing of sleepiness and wakefulness. These processes create a predictable pattern of two sleepiness peaks, which commonly occur about 12 hours after the midsleep period (during the afternoon for most people who sleep at night) and before the next consolidated sleep period (most commonly at night, before bedtime) (Richardson et al., 1982; see figure 1). Sleep and wakefulness also are influenced by the light/dark cycle, which in humans most often means wakefulness during daylight and sleep during darkness. People whose sleep is out of phase with this cycle, such as night workers, air crews, and travelers who cross several time zones, can experience sleep loss and sleep disruption that reduce alertness (Åkerstedt, 1995b; Samel et al., 1995). The panel noted that the sleep-wake cycle is intrinsic and inevitable, not a pattern to which people voluntarily adhere or can decide to ignore. Despite the tendency of society today to give sleep less priority than other activities, sleepiness and performance impairment are neurobiological responses of the human brain to sleep deprivation. Training, occupation, education, motivation, skill level, and intelligence exert no influence on reducing the need for sleep. Micro-sleeps, or involuntary intrusions of sleep or near sleep, can overcome the best intentions to remain awake. SLEEPINESS IMPAIRS PERFORMANCE Sleepiness leads to crashes because it impairs elements of human performance that are critical to safe driving (Dinges, Kribbs, 1991). Relevant impairments identified in laboratory and in-vehicle studies include: Slower reaction time. Sleepiness reduces optimum reaction times, and moderately sleepy people can have a performance- impairing increase in reaction time that will hinder stopping in time to avoid a collision (Dinges, 1995). Even small decrements in reaction time can have a profound effect on crash risk, particularly at high speeds. Reduced vigilance. Performance on attention- based tasks declines with sleepiness, including increased periods of nonresponding or delayed responding. Deficits in information processing. Processing and integrating information takes longer, the accuracy of short-term memory decreases, and performance declines (Dinges, 1995). Often, people use physical activity and dietary stimulants to cope with sleep loss, masking their level of sleepiness. However, when they sit still, perform repetitive tasks (such as driving long distances), get bored, or let down their coping defenses, sleep comes quickly (Mitler et al., 1988; National Transportation Safety Board, 1995). THE CAUSES OF SLEEPINESS/DROWSY DRIVING Although alcohol and some medications can independently induce sleepiness, the primary causes of sleepiness and drowsy driving in people without sleep disorders are sleep restriction and sleep fragmentation. Sleep restriction or loss. Short duration of sleep appears to have the greatest negative effects on alertness (Rosenthal et al., 1993a; Gillberg, 1995). Although the need for sleep varies among individuals, sleeping 8 hours per 24-hour period is common, and 7 to 9 hours is needed to optimize performance (Carskadon, Roth, 1991). Experimental evidence shows that sleeping less than 4 consolidated hours per night impairs performance on vigilance tasks (Naitoh, 1992). Acute sleep loss, even the loss of one night of sleep, results in extreme sleepiness (Carskadon, 1993b). The effects of sleep loss are cumulative (Carskadon, Dement, 1981). Regularly losing 1 to 2 hours of sleep a night can create a "sleep debt" and lead to chronic sleepiness over time. Only sleep can reduce sleep debt. In a recent study, people whose sleep was restricted to 4 to 5 hours per night for 1 week needed two full nights of sleep to recover vigilance, performance, and normal mood (Dinges et al., 1997). Both external and internal factors can lead to a restriction in the time available for sleep. External factors, some beyond the individual's control, include work hours, job and family responsibilities, and school bus or school opening times. Internal or personal factors sometimes are involuntary, such as a medication effect that interrupts sleep. Often, however, reasons for sleep restriction represent a lifestyle choice-sleeping less to have more time to work, study, socialize, or engage in other activities. Job-Related Sleep Restriction. Contemporary society functions 24 hours a day. Economic pressures and the global economy place increased demands on many people to work instead of sleep, and work hours and demands are a major cause of sleep loss. For example, respondents to the New York State survey who reported drowsy-driving incidents cited a variety of reasons related to work patterns. These included working more than one job, working extended shifts (day plus evening plus night), and working many hours a week (McCartt et al., 1996). Personal Demands and Lifestyle Choices. Many Americans do not get the sleep they need because their schedules do not allow adequate time for it. Juggling work and family responsibilities, combining work and education, and making time for enjoyable pastimes often leave little time left over for sleeping. Many Americans are unaware of the negative effects this choice can have on health and functioning (Mitler et al., 1988). From high-profile politicians and celebrities to the general population, people often see sleep as a luxury. One in four respondents who reported sleeping difficulties in a recent Gallup Survey said you cannot be successful in a career and get enough sleep (National Sleep Foundation, 1995). Sleep fragmentation. Sleep is an active process, and adequate time in bed does not mean that adequate sleep has been obtained. Sleep disruption and fragmentation cause inadequate sleep and can negatively affect functioning (Dinges, 1995). Similar to sleep restriction, sleep fragmentation can have internal and external causes. The primary internal cause is illness, including untreated sleep disorders. Externally, disturbances such as noise, children, activity and lights, a restless spouse, or job-related duties (e.g., workers who are on call) can interrupt and reduce the quality and quantity of sleep. Studies of commercial vehicle drivers present similar findings. For example, the National Transportation Safety Board (1995) concluded that the critical factors in predicting crashes related to sleepiness (which this report called "fatigue") were duration of the most recent sleep period, the amount of sleep in the previous 24 hours, and fragmented sleep patterns. Circadian factors. As noted earlier, the circadian pacemaker regularly produces feelings of sleepiness during the afternoon and evening, even among people who are not sleep deprived (Dinges, 1995). Shift work also can disturb sleep by interfering with circadian sleep patterns. EVALUATING SLEEPINESS An ideal measure of sleepiness would be a physiologically based screening tool that is rapid and suitable for repeated administration (Mitler, Miller, 1996). No measures currently exist for measuring sleepiness in the immediacy of crash situations. Furthermore, a crash is likely to be an altering circumstance. A measuring system would be performance based and in vehicle, linked to alerting devices designed to prevent the driver from falling asleep. The current tools for the assessment of sleepiness are based on questionnaires and electrophysiological measures of sleep, and there is interest in vehicle-based monitors. A comprehensive review of these efforts is beyond the scope of the present report. In the following brief discussion, some tools for the assessment of sleepiness are described to illustrate the different subjective and objective measures of chronic and situational (acute) sleepiness and the vehicle-based technology to sense sleepiness. Assessment for chronic sleepiness. The Epworth Sleepiness Scale (ESS) (Johns, 1991) is an eight-item, self-report measure that quantifies individuals' sleepiness by their tendency to fall asleep "in your usual way of life in recent times" in situations like sitting and reading, watching TV, and sitting in a car that is stopped for traffic. People scoring 10 to 14 are rated as moderately sleepy, whereas a rating of 15 or greater indicates severe sleepiness. The ESS is not designed to be used to assess situational sleepiness or to measure sleepiness in response to an acute sleep loss. The ESS has been used in research on driver sleepiness and in correlations of sleepiness to driving performance in people with medical disorders. Other rating tools that measure an individual's experience with sleepiness over an extended period of time and contain a component or scale that is congruent with measuring sleepiness include the Pittsburgh Sleep Quality Index (Buysse et al., 1989) and the Sleep-Wake Activity Inventory (Rosenthal et al., 1993b). Other self-report instruments obtain historical information pertinent to sleepiness using patient logs and sleep-wake diaries (Douglas et al., 1990) and the Sleep Disorders Questionnaire (Douglas et al., 1994). The information gathered with these instruments has not been as widely applied to assessments of noncommercial crashes. Laboratory tools for measuring sleepiness include the Multiple Sleep Latency Test (MSLT) (Carskadon et al., 1986; Carskadon, Dement, 1987) and the Maintenance of Wakefulness Test (MWT) (Mitler et al., 1982). The MSLT mea- sures the tendency to fall asleep in a standardized sleep-promoting situation during four or five 20-minute nap opportunities that are spaced 2 hours apart throughout the day and in which the individual is instructed to try to fall asleep. Sleep is determined by predefined brain wave sleep-staging criteria. The presumption under-lying this test is that people who fall asleep faster are sleepier. Individuals who fall asleep in 5 minutes or less are considered pathologically sleepy; taking 10 minutes or more to fall asleep is considered normal. In the MWT, individuals are instructed to remain awake, and the time it takes (if ever) in 20 minutes to fall asleep by brain wave criteria is the measure of sleepiness. Although the relative risk for fall-asleep crashes has not been established, individuals who exhibit a sleep latency of less than 15 minutes on the MWT are categorically too sleepy to drive a motor vehicle (Mitler, Miller, 1996). The MSLT and MWT were developed for neuro- physiologic assessment and are sensitive to acute as well as chronic sleep loss. Both assume standardization of procedures involving specially trained personnel and are not valid if the individual being tested is ill or in pain (Carskadon, 1993b). The panel thought that the use of these medical tests may not be practical for crash assessment; however, the use of a modified "nap test" has been used along with questionnaires for field assessment of driver sleepiness (Philip et al., 1997). Assessment for acute sleepiness. Acute sleepiness is defined as a need for sleep that is present at a particular point in time. The Stanford Sleepiness Scale (SSS) (Hoddes et al., 1973) is an instrument that contains seven statements through which people rate their current level of alertness (e.g., 1= "feeling...wide awake" to 7= "...sleep onset soon..."). The scale correlates with standard performance measures, is sensitive to sleep loss, and can be administered repeatedly throughout a 24-hour period. In some situations, the scale does not appear to correlate well with behavioral indicators of sleepiness; in other words, people with obvious signs of sleepiness have chosen ratings 1 or 2. The Karolinska Sleep Diary (Åkerstedt et al., 1994) contains questions relating to self-reports of the quality of sleep. Laboratory and some field studies suggest that most subjective sleep measures in this scale show strong covariation and relation to sleep continuity across a wide spectrum of prior sleep length and fragmentation. As in the SSS, several questions are asked to determine values for subjective sleepiness. A Visual Analogue Scale (VAS) for sleepiness permits the subjects to rate their "sleepiness" in a continuum along a 100-mm line (Wewers, Low, 1990). Anchors for sleepiness range from "just about asleep" (left end) to "as wide awake as I can be" (right end). Persons rate their current feelings by placing a mark on the line that indicates how sleepy they are feeling. The VAS is scored by measuring the distance in millimeters from one end of the scale to the mark placed on the line. The VAS is convenient and rapidly administered over repeated measurements. In all these attempts to measure subjective sleepiness, a person's response is dependent on both the presentation of the instructions and the subject's interpretation of those instructions. Problems related to these factors may confound interpretation between studies and between groups of different ages or cultures. Vehicle-based tools. There are some in-vehicle systems that are intended to measure sleepiness or some behavior associated with sleepiness in commercial and noncommercial driving. Examples include brain wave monitors, eye-closure monitors, devices that detect steering variance, and tracking devices that detect lane drift (Dinges, 1995). This technology is currently being examined in physiologic, psychophysiologic, and crash-prevention domains. There is insufficient evidence at present to judge its application and efficacy in regard to noncommercial driving.