Students who sleep 7 hours per night during the exam period score an average of 1.7 points higher (on a scale of 20) on their exams than peers who get only 6 hours of sleep. Researchers at Ghent University and KU Leuven surveyed 621 first-year students about the quality of their sleep during the exam period and correlated it with their exam performance.

In total, approximately 30% of students received a score of 5 on the Pittsburgh Sleep Quality Index (PSQI)–which earned them the label of “bad sleeper.”

More female students (35%) than male students (26%) fell in the “bad sleeper” category.

The researchers corrected their results to account for differences in socio-economic background and general health traits.

“All else equal, students who generally got a good night’s sleep performed better on exams,” says Dr Stijn Baert of Ghent University, in a release. “Students who fell one standard deviation (and thus significantly) above the average PSQI score achieved almost a full point less for each exam they sat.”

Additionally, the number of hours slept—and not the quality of sleep during that time–played a significant role in exam performance. “Students who increased their night’s sleep from 6 to 7 hours were rewarded with an average increase of 1.7 points (on a scale of 20) for each exam. Of course, it goes without saying that the optimal sleep time varies for each individual.”

By way of explaining the results, the researchers point to previous studies, which found that sleep is essential for one’s all-around state of mind and motivation. “A good night’s sleep optimizes cognitive performance in a very direct way since new knowledge is integrated into our existing knowledge base while we sleep,” says Baert. The process of memory consolidation occurs mostly during the REM sleep phase, which is concentrated in the second half of the sleep cycle. Getting at least 7 hours of sleep is essential to this, say the researchers.