If you are having trouble deciding between the couch or the health club, know this: After just a week or so of inactivity, your brain suffers.

That’s what researchers from the University of Maryland found when they set out to determine how long it takes before your brain begins to change from the lack of stimulation frequent exercise provides. As reported in The New York Times, studies show greater blood vessel, synapse, and neuron creation activity, with sturdier thinking and memory skills in people who exercise more.

More blood flow to the brain means more activity, which means more biochemical processing, scientists believe.

Exercise is thought to be particularly important for brain health because it appears to ramp up blood flow not only during the actual activity, but also throughout the day. Studies show when sedentary people began an exercise program, they soon improved blood flow to their brains, even when they were resting.

But just how long it takes for these positive effects to fade hasn't been put to the test.

So the researchers tested 12 competitive runners between the ages of 50 and 80 who regularly run 35 miles or more each week, and who had been running for at least 15 years. Their cognitive skills were put to the test before and after a 10-day rest period. MRI tests were utilized to track blood flow to the brain. During the rest period, the athletes did not run or exercise.

The results after the rest period were surprising. The runners recorded much less blood flow to their brains, though cognitive function did not seem lessened. But since one measure of healthy brain activity is blood flow, the researchers' message is clear: Skip the couch and head to the club.