From The Atlantic:
Just before the dawn of the twentieth century, William James, one of the early giants of modern psychology, explained that human attention comes in two different forms. The first is directed attention, which enables us to focus on demanding tasks like driving and writing. Reading a book also requires directed attention, and you'll notice that you start to zone out when you're tired, or when you've been reading for hours at a time. The second form is involuntary attention, which comes easily and doesn't require any mental effort at all. As James explained, “Strange things, moving things, wild animals, bright things, pretty things, words, blows, blood, etc., etc., etc.” all attract our attention involuntarily.
Nature restores mental functioning in the same way that food and water restore bodies. The business of everyday life — dodging traffic, making decisions and judgment calls, interacting with strangers — is depleting, and what man-made environments take away from us, nature gives back. There's something mystical and, you might say, unscientific about this claim, but its heart actually rests in what psychologists call attention restoration theory, or ART. According to ART, urban environments are draining because they force us to direct our attention to specific tasks (e.g., avoiding the onslaught of traffic) and grab our attention dynamically, compelling us to “look here!” before telling us to instead “look over there!” These demands are draining — and they're also absent in natural environments. Forests, streams, rivers, lakes, and oceans demand very little from us, though they're still engaging, ever changing, and attention-grabbing. The difference between natural and urban landscapes is how they command our attention. While man-made landscapes bombard us with stimulation, their natural counterparts give us the chance to think as much or as little as we'd like, and the opportunity to replenish exhausted mental resources.