Key factors include sleeping less than six hours per night, being awake for 20 hours or longer, working more than one job and/or working night shifts, and frequent driving between midnight and 6 a.m.
The study is different from previous studies because it looked at real-world crashes, according the AAA. Researchers used police crash reports and driver records to identify and interview 1,400 drivers. The sample included four groups: drivers who fell asleep, drivers who were fatigued, drivers who crashed for non-sleep reasons, and a control group of drivers who had not had a crash in three years.
Researchers had subjects fill out a detailed questionnaire about work schedules, sleep habits and the circumstances surrounding the crash. Drivers were also asked questions to assess their present levels of sleepiness.
Sleep and fatigue crash drivers had been awake longer and had slept less. Only a fifth reported getting eight or more hours of sleep before the crash, compared to nearly half of the control group. Drivers in sleep and fatigue crashes were also more likely to deal with their drowsiness once they were on the road, rather than planning ahead and taking precautions.
About half the drivers in the sleep-related crashes said they did not feel even moderately drowsy before they crashed.