While driving, William Nickerson constantly thinks about staying awake.

“It’s always in the front of my mind,” he said. “I couldn’t live with my conscience if I fell asleep at the wheel and killed a family.”

A new study by the Centers for Disease Control and Prevention reports that 4.2 percent of 147,076 people surveyed said they had fallen asleep at least one time while driving during the past 30 days. And some studies have estimated that as many as one-third of fatal crashes might involve drowsy drivers.

“Although it is clear that falling asleep while driving is dangerous, drowsiness impairs driving skills even if drivers manage to stay awake,” the CDC reports.

Nickerson, 58, of Goose Creek drives up to 2,000 miles per month for his job as area supervisor for the state Meat and Poultry Inspection Department of Clemson University.

As a young man, he could drive 12 hours at a stretch. Now, he limits himself to four hours on the road at one time. He also avoids driving at night.

“I’m very aware of being excessively sleepy and tired, especially while driving. I don’t want to be a danger to the public,” said Nickerson, who has obstructive sleep apnea, a serious disorder that occurs when a person’s breathing is interrupted.

Those at increased risk for drowsy driving include commercial drivers, night shift workers, people with untreated sleep disorders and anyone who does not get enough sleep, according to the CDC study described in the Jan. 4 issue of the agency’s Morbidity and Mortality Weekly Report.

A Dorchester County woman died in November after driving off the roadway and hitting a drain culvert. According to the coroner’s office, family members said she had a history of falling asleep while driving. Last March, a Goose Creek truck driver, 29, fell asleep while motoring in his 18-wheeler on Interstate 12 in Louisiana. No one was injured, but traffic was blocked for 12 hours after the truck fell on its right side, according to news reports.

Nickerson’s condition, which put him at risk for daytime sleepiness, was diagnosed 12 years ago when he sought medical help because of his wife’s complaints about his snoring and because he was tired during the day. People with untreated sleep apnea stop breathing repeatedly during the night, sometimes hundreds of times.

Obstructive sleep apnea happens when the soft tissue in the back of the throat collapses during sleep. Nickerson’s condition is managed with medication that keeps him awake and alert during the day. At night, he wears a continuous positive airway pressure mask that pumps air to keep his windpipe open so he can sleep restfully.

Dr. Michael Spandorfer, who treats Nickerson, is the sleep lab director at Bon Secours St. Francis Hospital. He said the risk to the public is the same for someone driving with untreated sleep apnea and a person motoring while intoxicated. In addition to an increased risk of impaired driving, untreated sleep apnea significantly raises the risk for heart attack and stroke, Spandorfer said.

He diagnosed Nickerson following the results of a sleep study. Nickerson’s family physician referred him to Spandorfer.

“I was smart enough not to ignore it,” Nickerson said.

Being sleepy slows reaction time, makes drivers less attentive and impairs decision-making, all of which can contribute to motor vehicle crashes. Sleep-related crashes often involve a single vehicle going off the road with no evidence of braking or other attempts to prevent the crash.

“Drowsy driving crashes are more likely to result in injuries and fatalities,” according to the CDC, which said that sleep-related crashes make up a disproportionate percentage of rear-end and head-on collisions.