In findings that give new meaning to the phrase “boob tube,” new research finds that watching TV is indeed bad for your brain.

Researchers who tracked 3,274 people whose average age was 25 for 25 years found those who watched them most TV — more than three hours a day most days — had the poorest performance on three tests of mental sharpness, The New York Times reports.

Compared with those who watched TV the least, those who watched the most were up to twice as likely to score poorly on the tests. Researchers ruled out other factors that might account for the differences, such as educational level, body mass index, smoking, alcohol use, hypertension, and diabetes.

Those with the lowest levels of physical activity and the highest levels of TV watching were the most likely to have poorest test results, according to the study, published in the journal JAMA Psychiatry.

“We can’t separate out what is going on with the TV watching,” said lead researcher Dr. Kristine Yaffe, a professor of psychiatry and neurology at the University of California-San Francisco. “Is it just the inactivity, or is there something about watching TV that’s the opposite of cognitive stimulation?”