People continue to drive even when they know they are sleepy, suggests a large study published on bmj.com today. This has important implications for public safety, say the researchers.

Studies have shown that sleepiness in drivers is an important factor contributing to the burden of traffic related injury and death. Estimates of the proportion of car crashes attributable to sleepiness range from 3% to 33%, but little is known about the extent to which drivers are able to assess that they are sleepy while driving.

Researchers in France examined the association between self reported driving while sleepy and the risk of serious road traffic accidents (RTAs) in 13,299 middle aged drivers.

They collected data on sleepiness and other driving behaviours in 2001, and serious RTAs in 2001-3. Socioeconomic status was recorded, and a range of other factors that could affect the results were taken into account.

In answer to the question “in the past 12 months, have you ever driven while sleepy?” 63% of participants responded never, 36% a few times in the year, 0.8% about once a month, 0.3% about once a week, and 0.2% more than once a week.

There was a strong association between self assessed driving while sleepy and the risk of serious road traffic accidents over the next three years. This risk increased with reported frequency of driving while sleepy.

For example, participants who reported driving while sleepy “a few times in the year” were 1.5 times more likely to have been involved in a serious RTA compared with those who reported not driving while sleepy over the same period. And those who reported doing so “once a month or more often” were nearly three times more likely to have been involved in a serious RTA.

Further analysis did not change this association and follow-up questionnaires in 2004 also found a similar trend.

These results clearly show that self assessed driving while sleepy is a powerful predictor of serious road traffic accidents, and suggest that drivers are aware that they are sleepy when driving but do not act accordingly, say the authors. Drivers may either underestimate the impact of sleepiness on their driving performance or overestimate their capacity to fight sleepiness.

Messages on prevention should therefore focus on convincing sleepy drivers to stop driving and sleep before resuming their journey, they conclude.