The science is in: when you go to bed and how consistent your sleeping patterns are matter — they matter a lot.

A study on MIT students showed that the time they go to bed, and the consistency of their sleep habits, are directly and strongly linked with the grades they’re getting. For all of you who were hoping that a good night’s sleep just before an exam is enough, sorry to break it to you — it’s not. But, consistent, quality rest for several nights in a row is enough to make a difference.

Putting the Z’s in A’s

“Of course, we knew already that more sleep would be beneficial to classroom performance, from a number of previous studies that relied on subjective measures like self-report surveys,” says Jeffrey Grossman, an MIT professor and co-author of the study.

“But in this study the benefits of sleep are correlated to performance in the context of a real-life college course, and driven by large amounts of objective data collection.”

The paper finds a strong link between how much rest students are getting and their overall academic performance. The team worked with 100 students in an MIT engineering class, who received Fitbits to wear and track their activity 24/7. In exchange, they allowed the researchers access to a semester’s worth of their academic performance and this activity data.

Initially, the team didn’t set out to study sleep at all. They wanted to see if there was any correlation between physical exercise and the academic performance of students in Grossman’s class 3.091 (Introduction to Solid-State Chemistry). As part of the study, a quarter of those 100 students who received Fitbits were enrolled in an intense fitness class MIT’s Department of Athletics, Physical Education, and Recreation, with the help of assistant professors Carrie Moore and Matthew Breen, who created the class specifically for this study.

The idea was that there might be a measurable performance difference in test grades between the group enrolled in this fitness class and their peers. However, the researchers were unable to find such a link.

“What we found at the end of the day was zero correlation with fitness, which I must say was disappointing since I believed, and still believe, there is a tremendous positive impact of exercise on cognitive performance,” Grossman says.

One possibility is that the time intervals between the fitness program and individual classes may have been too long for the effect to last. However, the Fitbits’ proprietary algorithms did detect periods of sleep and changes in sleep quality, primarily based on lack of activity.

A surprising finding of the paper was that individuals who went to bed after a particular threshold time — it tended to be 2 a.m. on average for the students here, but there were individual variations — tended to perform more poorly on tests no matter how much sleep they got overall. In other words, it wasn’t just sleep duration that mattered; when they got their rest also mattered.

The correlation was pretty in-your-face, Grossman adds. Essentially, there’s a straight-line relationship between the grades students got on the eleven quizzes, three midterms, and final exam they took during the study period (with the grades ranging from A’s to C’s).

“There’s lots of scatter, it’s a noisy plot, but it’s a straight line,” he says.

The link itself wasn’t surprising, but its strength was. While the results can’t prove beyond a doubt that sleep was the determining factor in the students’ performance, they are a very strong indication that sleep “really, really matters.”

The results also showed that students who got a good night’s sleep before a test saw no improvement in grades if they didn’t maintain sleep discipline over a longer period of time. Another finding is that, in Grossman’s words, “when you go to bed matters”.

“If you get a certain amount of sleep—let’s say seven hours—no matter when you get that sleep, as long as it’s before certain times, say you go to bed at 10, or at 12, or at 1, your performance is the same. But if you go to bed after 2, your performance starts to go down even if you get the same seven hours. So, quantity isn’t everything.”

Quality of sleep also mattered. For example, those who got relatively consistent amounts of sleep each night did better than those who had greater variations from one night to the next, even if they ended up with the same average amount.

The paper “Sleep quality, duration, and consistency are associated with better academic performance in college students” has been published in the journal Science of Learning.