I've been reading the book Brain Rules by neuroscientist John Medina. He takes the latest scientific research and understanding about how the brain works and puts it into plain english.
As a result of our evolutionary history, the brain has three layers. The first layer is the lizard brain. It cares about the four F's - fight, flight, food, and f___, um... reproduction. Next there's the mammalian layer which gives us our nurturing instincts and emotions. Finally, at the top of the brain pyramid sits the cortex - rational thought, planning, and impulse control.
Our brain was forged in an environment of constant motion. We hunted and gathered all day long. This has a major implication. Cognitive health is predicated on an active lifestyle. Statistics show that regular exercise cuts your risk of dementia and Alzheimer's in half.
Sharks have to keep moving just to breathe. We aren't quite that dependent on motion... but it's close. You don't need a crazy amount of exercise to get the cognitive health benefits. A couple of intense sessions a week and a daily walk does the trick. So hit it!