The U.S. Centers for Disease Control and Prevention (CDC) conducted a study of driving habits during 2009 and 2010 to find out how many people fall asleep while driving. Out of 147,076 respondents, 4.2% reported having fallen asleep while at the wheel during the previous 30 days.

The potential for drowsy driving decreases with age, according to the CDC’s findings, with 4.9% of adults ages 18-44 reporting having fallen asleep whereas only 1.7% out of those 65 years or older reported having fallen asleep. Respondents who were employed were more likely to report drowsy driving (at 5.1%).

The study also noted that drowsy driving was associated with other sleep issues. For example, adults who reported frequent insufficient sleep, a daily sleep duration of six hours or less, snoring or unintentionally falling asleep during the day, reported drowsy driving more frequently.

The CDC went on to say that drowsiness in general impairs driving skills, slowing reaction times and making drivers less attentive. Crashes related to sleep issues are more likely to happen at night or during mid-afternoon. The CDC said these crashes often have a single vehicle going off the road. Sleep related crashes also make up a disproportionate number of rear-end and head-on collisions and they are more likely to result in injuries and fatalities than other non-drowsy driving crashes.

The CDC suggested that drivers on the road who are falling asleep should pull over until fully rested. In addition, the CDC said data on techniques to stay awake, for example opening the window, turning up the radio, or turning up the air conditioner, are not effective.