Women, young adults and older adults are also more likely to suffer from excessive sleepiness, according to Zee. The new survey reflected those tendencies.
The study also showed being obese, which is a risk factor for sleep apnea, was a risk factor for excessive daytime sleepiness.
"Obesity is a big risk factor for sleep apnea. And a significant proportion of the population is obese," said Malhotra, though the differences between the United States and Europe "may be in part because there's less obesity and less sleep apnea. People tend to be thinner over there."
Sleep specialists say it is also difficult quantifying how sleepy a population is, because people may be unaware of how sleepy they truly are.
"The last thing a truck driver remembers before falling asleep on the road is being wide awake," said Dr. Mark Eric Dyken, director of the Sleep Disorders Center in Iowa City and associate professor at the University of Iowa.
"Some of the people in my clinic say, 'Oh, I'm not tired,' and then they conk out. It's because they don't know what it's like to have good sleep in our society," he said.