@markhenick This seems consistent with my anecdodal observations that extreme endurance sports, like mountaineering and ultra-trailrunning, seem to have disproportionate numbers of athletes who struggle with serious depression or are in recovery from addiction, both of which are associated with feelings of hopelessness.

That said, I'd like to see a more careful study of the effect. This one seems to have potentially conflated exposure to daylight with exercise, which is problematic since daylight and exercise are both known to have positive effects on mood. Also, with such a small sample size I wonder whether a paired-sample design exposing each patient to both treatments might have better controlled for other nuisance variables.

What I'd really like to see though, based on my observations of extreme sports, is an investigation into how exercise intensity modulates the effect of exercise on mood and well being. I suspect that any physical activity is better than nothing, but that the effect size increases with intensity. I also suspect that for some patients prolonged high-intensity exercise might have a significant positive effect when lower intensity exercise fails to do much. Maybe someone has already worked on that. I might have to dig around when I have time.