The ideal amount of homework to assign to middle school students is about an hour a day, according to new research in the Journal of Educational Psychology. The most benefit is derived, the study found, when teachers assign homework on a regular basis that students can complete on their own.

Researchers Ruben Fernandez-Alonso and Javier Suarez-Alvarez from Spain’s University of Oviedo asked 7,725 students with an average age of 13 attending public, state-subsidized and private schools in Spain to complete a survey on their homework habits. The researchers then examined the students’ standardized test scores in math and science.

Those students who spent about 90 to 100 minutes a day on homework scored highest on the math and science assessments; however, their scores were only slightly higher than those of students who spent 60 to 70 minutes on homework, suggesting diminishing returns. And students who spent more than 100 minutes a day on homework, in fact, scored lower on their tests.

The researchers found that students who did homework every day scored 50 points higher on a standardized math test than those who did it less frequently, and those who did their homework on their own scored 54 points higher than those who had outside help. Similar patterns were observed for science.

The researchers concluded that teachers should refrain from assigning more than 60 to 70 minutes of homework to middle school students because any more than that is an inefficient use of students’ time. They suggest that teachers should focus on assigning homework regularly so that students develop good work habits and learn how to engage in self-directed learning.