Children who watch television or play video games on school nights don't do as well in school.
A new study had middle school students rate their performance from "excellent" to "below average." The more TV they watched on weekdays, the worse they said they did.
But study co-author Dr. Iman Sharif says weekend viewing didn't seem to hurt their performance, unless it was more than four hours a day.
The study also found that children whose parents allowed them to watch R-rated movies also did worse in class, boys especially.
What do the experts do? Co-author Sharif has three children ages seven, eleven and 15, and says her children watch about an hour of TV after school and then it goes off.
The study appears in the Journal Pediatrics.