Aug. 12 (HealthDay News) -- U.S. college students don't get enough sleep, and stress is the prime reason, a new study reports.

About 68 percent of college students who were surveyed said that worries about school and life keep them awake, with one-fifth saying this occurs at least once a week. The study, which appears online in the Journal of Adolescent Health, found that less than a third of the 1,125 survey participants get the eight hours of sleep at night that people their age need.

"Students underestimate the importance of sleep in their daily lives," study co-author Roxanne Prichard, an assistant professor of psychology at the University of St. Thomas, in St. Paul, Minn., said in a news release from the journal's publisher. "They forgo sleep during periods of stress, not realizing that they are sabotaging their physical and mental health."

Lack of sleep can cause problems with a person's immune and cardiovascular system and increase the likelihood of other health risks, such as weight gain, she said.

About three in five of the students said they have irregular sleep-wake patterns, and many said they use drugs or alcohol regularly to help them either sleep or stay alert, the survey found. The regular use of stimulants and sedatives can increase the chance of becoming addicted to them.

Weekday all-nighters are pulled at least once a month by 20 percent of those polled, and 35 percent said they stayed up until 3 a.m. at least once a week. Skipping three or more classes in a month or falling asleep in class was common among 12 percent of the poor sleepers, the researchers noted.

SOURCE: Journal of Adolescent Health, news release, Aug. 10, 2009