Scientists are finding that night shift work comes with a range of particular health risks, from heart disease to diabetes to breast cancer. This month, another study joined the pack — this one on the risk of traffic crashes among those who head home from work at sunrise.

To conduct the study, researchers from Brigham and Women’s Hospital and the Liberty Mutual Research Institute recruited and examined the experiences of 16 night shift workers as they participated in two two-hour daytime drives on a closed test track. The study participants undertook one driving test after working a night shift and another after a typical night’s sleep. Researchers found that more than 37 percent of the post-night shift drives were involved in a near-crash event, while nearly 44 percent of such drives were ended early for safety reasons. However, researchers documented zero near-crashes during post-sleep drives and didn’t have to end any of the post-sleep drives early for safety concerns. The study was published this month in the Proceedings of the National Academy of Sciences.

In noting that drowsiness is likely one of the most prevalent causes of preventable traffic crashes worldwide, study researchers Michael Lee, Mark Howard, William Horrey, Yulan Liang, Clare Anderson, Michael Shreeve, Conor O’Brien and Charles Czeisler write:

The nation’s 9.5 million shift workers, comprising 15% of the workforce, are at particular risk of drowsy driving. Night-shift work increases the risk for drowsy driving crashes, especially on the morning commute home from overnight work, when elevated homeostatic sleep pressure interacts with the peak of circadian sleep propensity to create a critical zone of performance vulnerability.

According to the study, in more than one-third of the post-night shift drives, a safety observer had to initiate an emergency braking tactic due to a near-crash event. Lane excursions, slow eye movement and microsleep episodes (sleep for less than 3 seconds) were much more frequent during the test drives after a night shift, as were ocular measurements of drowsiness, the study found. In fact, researchers documented indicators of impaired driving in just the first 15 minutes of driving following a night shift, with a marked increase in the rate of slow eye movements and drowsiness after 30 minutes of driving. Researchers described impairments as “severe” after 45 minutes of driving. And even longtime night shift workers were vulnerable to impaired driving. The researchers write:

Driving following night-shift work puts drivers as well as other road users at elevated risk of motor vehicle crashes. The results from the present study, using actual night-shift workers and carried out in an instrumented vehicle, demonstrate the contribution of sleep-related impairment of neurobehavioral function to the increased crash risk found in drivers commuting home after overnight work and contribute new and compelling evidence of these risks. …These data therefore indicate that even night-shift workers with a relatively brief commute are vulnerable to fatigue-related driving incidents.

Unfortunately, many night shift workers probably have few, if any, commuting alternatives. However, given the study’s results, the researchers advised night shift workers to limit their post-work driving durations, seek other commuting options and, perhaps most importantly, learn how to recognize the signs of increased drowsiness and when it’s time to stop driving.

“The number of near-crash events that occurred during the study starkly emphasized the statistics of nearly half a million crashes and 6,500 fatalities annually that directly result from driver fatigue,” said study author Horrey, principal research scientist at the Liberty Mutual Research Institute for Safety, in a news release. “While we are all generally aware of the risks associated with drowsy driving, these outcomes really underscored just how dangerous a homeward commute can be for this working population.”

To download a full copy of the study, visit the Proceedings of the National Academy of Sciences.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.