in Thoreau’s Journal:  

We must go out and re-ally ourselves to nature every day. We must make root, send out some little fibre at least, even every winter day.  I am sensible that I am imbibing health when I open my mouth to the wind. Staying in the house breeds a sort of insanity always. Every house is, in this sense, a sort of hospital. A night and a forenoon is as much confinement to those wards as I can stand. I am aware that I recover some sanity, which I had lost, almost the instant that I come abroad.