CHP Officer Jon Sloat thought he'd spotted a drunken driver.
The car was weaving back and forth across three lanes on the Cotati Grade of Highway 101 before Sloat, off duty at the time, pulled him over.
But the man wasn't drunk.
"He'd worked all night," Sloat said. "He was exhausted and weaving all over the place."
One in 24 adults admitted to recently falling asleep behind the wheel, according to a study released last week by the federal Centers for Disease Control and Prevention.
In Sonoma County, 112 people were injured in 193 crashes in which fatigue was a factor from 2008 through 2010, according to the latest figures available from the CHP. Three people were killed.
"Drowsy driving can be just as dangerous as drunk driving," Sloat said.
For the survey, researchers interviewed 147,000 people in 2009 and 2010 in California and 18 other states as well as the District of Columbia about sleeping and driving experiences.
Just more than 4 percent of adults said they fell asleep at the wheel during the previous 30 days.
Drowsy driving was more common among people who get less than six hours of sleep each night as well as those who snore.