When selecting your university classes next year, it might be wise to avoid those that begin too early.

A new study has found a direct correlation between getting up early and poor grades. Researchers at Singapore’s Duke-NUS Medical School analyzed data collected from tens of thousands of students and found those with early classes were overtired and received poorer grades than those who started later in the day, according to findings published in the journal Nature Human Behavior.

“If the goal of formal education is to position our students to succeed in the classroom and workforce, why are we forcing many university students into the bad decision of either skipping morning class to sleep more or attending class while sleep-deprived?” says Associate Professor Robert Gooley, one of the study’s authors. “The take-home message from our study is that universities should reconsider mandatory early morning classes.”

What’s the most difficult class you’ve taken? What made it so tough?