After long days of sedentary activity and staring at screens or textbooks, it can be especially important to stretch those muscles and get oxygen flowing to the brain. Our minds often need a breather as much as our bodies. While a healthy exercise plan is important, don't skip physical activity if you can't make your plan on a certain day. Get up and throw a football around, do some jumping jacks, or go for a brisk walk around the block before you sit back down to your work. Getting your heart rate up doesn't have to be fancy, but it is important.