I've recently gotten into intermittent fasting, and I've liked the results. The cold exposure one is interesting, though. My friend had a theory, which I've tested whenever I've been home in Finland, where I've exposed myself to harsh cold regularly to see if it has an effect on my mood.
My friend's theory is that because our inner temperature control is connected to our mood control, the way to beat a dark winter in a country like Finland is to suffer enough cold.
I did feel like it was working when I tried it.
Your friend's theory is interesting. Did he put it in words somewhere, or does he have a link to research regarding cold exposure and mood?
He reads a lot, so I took him by his word, but I can ask him the next time I talk to him.
please do :)