Here is a little information about the Lebensreform in English. https://en.m.wikipedia.org/wiki/Lebensreform
I am referring particularly to the time before WWI. (edit)
It is hard to say, of course, that Heidegger is "simply" anything, but part of this movement seems to have a very "pre-Socratic" nature to me. This seems to be a very "Greek" time in Germany to me.
Single sex, but also many mixed sex photographs of the time show young people are often nude, and simple. Loosely they are doing some gymnastics, but really they are free and just having fun in nature, at camp.
I am not talking about the hiking and folk elements so much as I am the nudity at camp, the fun the young people had, somehow I associate this with merely "being". The enjoyment of pure being.
I do not suggest that Heidegger cavorted around in the nude :). Perhaps he did. But he might have heard about it or saw photographs? This was the spirit of the times and Heidegger was rather young himself then.
With all the journals of philosophy and books printed each year, I wonder if this hypothesis that Heidegger was a Lebensreform (er) has been put forth? Or that he was influenced by this movement?
NB: when I say a "Greek time" or period in Germany, I mean such a time mainly for the younger people.