In the United States, traffic crashes involving commercial trucks kill more than 5,000 people and injure more than 110,000 each year. Studies show drowsy truck drivers are a factor in some of those crashes. In fact, "driver fatigue was recently judged to be the number-one problem in commercial transportation." Ten years ago (1988), Congress requested that the Federal Highway Administration (FHWA) study the problem of tired truckers in relation to federal regulations limiting their daily and weekly driving time. The study's results were published in "The Sleep of Long-Haul Truck Drivers" (The New England Journal of Medicine, September 11, 1997). Authors Merrill M. Mitler, James C. Miller, Jeffrey J. Lipsitz, James K. Walsh, and C. Dennis Wylie concluded that all long-haul truckers who participated in the study "obtained less sleep than is required for alertness on the job" and were most vulnerable to drowsiness or falling asleep late at night or early in the morning.

METHODS

The study took place from June to December of 1993. Eighty male, licensed commercial truck drivers voluntarily participated: 40 from the United States and 40 from Canada. Each trucker signed a government-approved consent form and was free to rest, nap, or withdraw from the study at any time. Participation was not based on financial or other incentives. Truckers also completed a questionnaire about their sleep habits, including the amount of sleep they felt was ideal for them. Their responses to the latter were similar to those in a Gallup survey of the general U.S. population--about 7 hours a night, plus or minus an hour.

In both the U.S. and Canada, truckers are permitted a maximum of 15 hours on duty per day, with a minimum 8 hours off duty. Truckers can drive only 10 hours without having 8 hours off in the U.S., and 13 hours without 8 off in Canada. The study used four of the most rigorous driving schedules permitted in the U.S. and Canada--two schedules from the route between St. Louis and Kansas City and two from the route between Toronto and Montreal. Schedules included both day and night driving. Each truck had a video camera to record the driver's face and the road and a computer to record the truck's speed and position.

Drivers chose their own bedtimes and rising times, according to their schedules. Their longest sleep in a 24-hour period was defined as "the principal period of sleep." They slept in rooms near their routes. Sixty to ninety minutes before their first sleep period, researchers fitted the truckers with electrodes that measured various physiologic functions, including eye movement and respiration. After the truckers awoke, researchers removed the respiration electrodes, but other electrodes continued to monitor physiologic functions while the truckers drove.

FINDINGS

Each driver had five principal sleep periods (400 total for the 80 drivers). The four schedules usually allowed for 8 hours off duty; however, 12 percent of the time drivers had less than 8 hours of off-duty time. Both regular sleep periods and naps were included in a trucker's total sleep time. Drivers took from 0-3 naps per day, and almost half (35) took at least one nap. Time spent napping increased the total amount of sleep by an average of three-quarters of an hour, plus or minus a half hour. Among all 80 drivers, the average time spent in bed was just over five-and-a-quarter hours (5.18 hours). The longest times in bed were for drivers on the day schedule, and the shortest on the night schedule. Younger drivers, whose average age was 36, spent more time in bed (including more naps) than older drivers, whose average age was 50.

Drivers slept an average of only 4.78 hours--2 or more hours less than their reported ideal amount of sleep. Schedules had a "significant effect" on how long drivers slept in a given period. On average, daytime drivers slept for longer periods than night drivers (5.38 hours vs. 3.83 hours).

In looking at episodes of drowsiness or falling asleep while driving, the study focused on those times when trucks traveled faster than 45 miles per hour (72 km per hour). Researchers watched for drivers with episodes of drowsiness equal to Stage 1 sleep ("the lightest stage of non-rapid-eye-movement sleep"). Researchers made 29,310 six-minute video recordings of drivers' faces, and 7 percent of the recordings showed a drowsy driver. More than half (56 percent) the drivers were drowsy in at least one recording, but 54 percent of the recordings showing a drowsy driver involved just eight drivers, all on either the evening or night schedules. Of the drowsy episodes recorded, 83 percent were during the evening and night (between 7:00 p.m. and 6:59 a.m.).

As expected, none of the truckers was involved in a crash. Typically, a truck tractor pulling one or more trailers is involved in a crash about every 1.2 million miles (1.9 million km). The trucks in the study drove about 204,000 miles (327,000 km).

LIMITATIONS AND RELATED SLEEP-RESEARCH

The study may have been limited because the truckers reported their ideal amounts of sleep, as opposed to researchers recording their sleep when they were not at work. Still, most reported sleeping more than 5 hours a night. The number of truckers involved may also have limited the study, since "each driver could not be studied on all schedules." However, researchers felt the four groups studied "were comparable."

The average 4.78 hours of sleep from the study was "much shorter than most standards." Sleep research shows the chances of falling asleep during normal waking hours increase if a person sleeps less than six hours and has "successive days" of too little sleep. Not enough sleep leads to more errors and inattentiveness and diminished psychomotor skills. Research also shows that "night driving after relatively little sleep is a better predictor of fatigue-related accidents than is night driving alone."

CONCLUSIONS AND RECOMMENDATIONS

Researchers found the truckers' short sleep times "disturbing" and noted those times were partly driver choice. They also pointed out that "the short durations of sleep--the inevitable result of short times in bed--probably explain the findings suggesting that the drivers got too little sleep." The researchers "found high rates of sleep efficiency (the ratio of the time asleep to the time spent in bed), an observation that is consistently reported in sleep-restriction studies and indicates an increased tendency to sleep." The increase in nighttime episodes of drowsy driving "was consistent with the expected effects of . . . decreased stimuli during night driving, and too little sleep." Because none of the recorded drowsy episodes involved a crash, "the drivers were probably drowsy during these episodes but not actually asleep." During true sleep, a driver would not respond to stimuli, which "would preclude safe driving."