More than a half-century later, I can still distinctly remember how my grandmother didn't hold back on advice. "Wear a hat so you don't get a chill and die" was often heard. She would also holler at me during the day, "Turn off the TV, go running outside so your mind does not rot." Then after dinner, she'd always say, "Go to sleep so you do well in school."  I didn't take her hat advice seriously, but she was onto something when it came to TV and sleep.

A recent study in the journal Lancet Child and Adolescent Health found kids who spent more than two hours a day of screen time performed worse on cognitive tests compared to peers who spent less time. These cognitive skills included memory, processing speed, language, and attention span.

The study looked at how 4,500 U.S. children from ages 8 to 11 at 20 study sites met the Canadian 24-Hour Movement Guidelines for Children and Youth. The participating children were asked to have active play at least one hour daily, less than two hours of recreational screen time daily, and over 10 hours of sleep each night. The children did best on a test of global thinking if they met all three objectives, but adequate sleep helped and limited recreational screen time helped the most.

The children who met the three objectives performed around 5 percent better on the tests than the average child. The researchers were able to determine screen time as the likely main factor. Kids who had less than two hours of screen time a day performed around 4 percent better than the average among their group not taking into account other habits.

There is always the possibility that these objectives do not cause better intellectual activity, but that children who do better in school naturally are more inclined to run around, not look at TV or video games as often, and have better sleep habits. So the cause and effect might just be related and not causal.

However, a relationship could exist considering the exposure of screen time on the brain especially before bedtime and how sedentary screen watching often replaces time which exercising, sleeping, or more mentally challenging activities could be taking place.

Logically, one can pay better attention in everyday life and school if not sleepy, and the natural need to blow off steam has been done so one can sit still. In my own experience as a pediatrician, I've seen how excessive screen time, lack of sleep, and not running around make children quite irritable and simply not fun to be around. I always ask about sleep and screen time as possible culprits when parents mention behavioral concerns.

While the advice to limit screen time and maintain healthy sleep habits isn't new, this study shows us that it helps children make the most of their brain power.