Sausages give you cancer. Kissing gives you cold sores. Drinking too much makes you fall down. Almost all of life's pleasures turn out to be bad for you. Which is why, if you ever meet Dr Eeva Karjalainen, you should shake her Nordic hand. According to Karjalainen and colleagues at the Finnish Forest Research Institute, one of the most enjoyable activities around can reduce stress and depression, ease muscle tension, counter attention deficit disorder, even calm an erratic heart. What is this wonder therapy? A walk in the woods. As Karjalainen puts it, "Many people feel relaxed and good when they are out in nature. But not many of us know there is also scientific evidence about the healing effects of nature."

I don't know about muscle tension, attention deficit or heart rates. But as far as stress and depression are concerned, the doctor's on to something.

I'm lucky enough to live surrounded by woods. A few paces puts me among spruce and birch and willow. There are deer and squirrels and stray cats. In winter, the bare branches grow furry with frost. They sparkle in the sun, and you're afraid to touch them in case they shatter.

It's lovely, but it's not exceptional, though when you live in a city you may forget such things exists. Britain is one of the least forested countries in Europe, but woods still cover 12% of the countryside, and half of that is open to the public. According to the Woodland Trust, almost two-thirds of us live within 4km of a wood that covers at least 200,000 square metres. And you don't need a giant forest to take a holiday from the world. To enter even the smallest wood is to cut yourself off from work and home.

Within those walls of bark, it's just you, the wildlife and anyone you trust enough to invite along. Try it on your own, if you can. You may get lost – last year I spent an hour and a half trying to escape an overgrown copse right next to the M25 – but you probably won't die. Savour the silence, if that's what nature serves up, or thrill to the tap-tap of the woodpecker. It's time to wake up and smell the sap.