"As screen time goes up, school performance goes down, but that does not happen overnight. If I watch a lot of TV today, I do not get bad marks in my class tomorrow," said Douglas Gentile, an associate professor of psychology at Iowa State University.
It may take up to seven months to see the benefits of limiting screen time of children, the study found.
"Parents are not likely to notice that putting limits on the children's media is having these effects seven months later," Gentile added.
Children got more sleep if parents limited screen time, which also resulted in lower risk of obesity.
"Parents limiting exposure to violent media resulted in increased pro-social behaviour and lowered aggressive behaviour seven months later," the researchers noted.
For the study, researchers analysed media habits of more than 1,300 school children who were recruited to participate in an obesity prevention programme.
Students and parents were surveyed about everything from screen time limits, to violent media exposure, to bed times and behaviour.
Teachers reported grades and commented on student behaviour and school nurses measured each student's height and weight, said the study appeared in JAMA Pediatrics.