On August 25, 1916, President Woodrow Wilson signed the act creating the National Park Service, providing protection for 35 established parks. Today, there are 407 — from the most-visited Great Smoky Mountains National Park, with 10 million visitors a year, to the least-visited Gates of the Arctic National Park and Preserve in Alaska, which gets only around 11,000 visitors annually.

Most people know that getting into any park (even if it's just part of a city block) is good for you. And science backs that up. A multicenter U.S.-Canada study found that "knowing and experiencing nature makes us healthier, happier people."

In addition, a University of Rochester study found that being around green spaces not only reduces stress and makes you feel better, but makes you behave better, too.

How much green time do you need? Australian researchers say 30 minutes a week minimum is what it takes to relax and reduce your risk for high blood pressure and depression. If everyone did that, they say, the prevalence of high blood pressure would decrease 9 percent, and depression by 7 percent.

Our recommendation: Spend 30 to 60 minutes (with a pedometer and a buddy) walking, five times a week. Head for 10,000 steps daily — outside, when weather permits.

You'll avoid what's being called Nature Deficient Disorder, which is afflicting everyone from screen-bound kids to housebound elders.