CANTON - It's tempting for college students to avoid those early-morning classes by rationalizing that they'll get more sleep and therefore, better grades.

A study by psychologists at St. Lawrence University shows that students starting later in the morning do, indeed, get more sleep, but they also tend to abuse alcohol more and wind up with lower grade-point averages than their early-bird classmates.

The study, by psychology professors Serge Onyper and Pamela Thacher, found that students who had later class times generally got more sleep, but also had more time to go out with friends. On the other hand, it appears that when students know they have an early class, they may tend to avoid nights out on the town.

Thacher says that later class start times might factor into the choices students make. "Those who choose later classes also tend to sleep longer and consume more alcohol and other substances," she says, "while those who elect earlier classes may be more motivated to find ways to offset the early start time by making healthier choices about their daily living."

She notes that while later class times predicted only slightly lower student grade-point averages, there is no question that later classes were associated with more drinking. Excessive alcohol consumption is the main negative influence on academic performance in college.

"The effects of later class-start times might include more sleep," Thacher says. "But this might be offset by lower quality sleep, which in turn might affect students' ability to engage, intellectually, with their coursework."

Onyper speculates that drinking more alcohol, known to disrupt sleep, may reduce the benefits of getting more sleep.

The psychology professors surveyed 253 college students who completed cognitive tasks and a one-week retrospective sleep diary, as well as questionnaires about sleep, class schedules, substance use and mood.

Thacher, author of a 2007 study showing that college students who pull "all-nighters" get lower grades, said that the study changed her mind about how to schedule classes. "Prior to this study, I advocated having classes start later in the morning, so that students could get more sleep," she said, noting that in that respect, the results are similar to those conducted on high school and junior high students. "But now, I would say that 8 or 8:30 a.m. classes are probably, for some students, going to be a much better choice."

The authors presented the results of the study at a recent sleep conference and submitted the study for publication to a scientific journal.