Hardly surprising given the long accepted notion that being surrounded by nature has a calming, restorative effect. The differential toxins and improved air quality in a natural versus unnatural environment are undoubtedly a factor. A green space outside your home may also mean less time inside. And as we now know, modern indoor environments are typically more toxic than the outdoors.

There may be an even better explanation for the positive correlation - grounding. Once the realm of quackery, earth grounding therapy is gaining scientific acceptance. Daily exposure to greenery increases the likelihood of physical contact between skin and ground. That physical (i.e.. barefoot) contact, which is thought to induce micro currents, has been shown to decrease inflammation, bolster immunity, heal wounds, reduce pain and improve sleep. 

It's now a balmy 5 degrees in the sun as we enjoy our spinach frittata while grounding ourselves on the patio...in open defiance of paraskevidekatriaphobia.