[freyr]

The cited scientific study was actually performed on nocturnal animals (rats), and the scientists found that long exposure to light caused the rats stress:

"...the brain of nocturnal rats generates a stress response to a long-day photoperiod, contributing to depression..."

Somebody must have extrapolated from this result that, since humans are diurnal, long periods of darkness would cause us stress and depression.

There is no direct mention of Vitamin K (or for that matter, any healthful benefits of the sun) in the cited study.