[jollybean]

Came here to say this:

VD may be it, or even indirectly a measure of 'not getting outside' enough.

Even the sunlight may not be it, literally a walk in the woods might be providing the effect.

Or rather 'spending too much time in our own minds' etc..

[digisign]

Bright, direct light seems to be a major component, which is not what most people think of being in the "woods." See:

https://news.ycombinator.com/item?id=31306477