This is definitely a real phenomenon. For me, it definitely has nothing to do with sun, since I live in Florida and get lots of sun all year long.
When I arrived in Italy about 2 weeks ago, after an overnight flight, during which I did not sleep at all (so in effect, I had a 48 hr day) with jet lag and the usual stress of traveling, something bizarre happened; my cognition suddenly got much better. Normally, my memory is total garbage, and I have to have redundant systems in place (reminders on phone, alarms, notes, etc... to remember most things)
Days before we left, I couldn't remember the Italian I'd learned years ago but as soon as I arrived, I was firing on all cylinders, picking up new vocab, phrases, and verb tenses like crazy. It was like all of a sudden my memory worked again. It felt like I had a completely different brain. Even before I was sick foreign language acquisition was a major weakness for me. Now suddenly it seems to be a strength.
Perhaps there is an increase in cortisol which reduced brain inflammation. I have no way to know if this is a viable explanation. I do tend to feel better after not sleeping (though this is true only for one night of missed sleep, it doesn't hold true for long term sleep deprivation).
So far, I'm not feeling any better physically (though when traveling, I often do). I fully expect that my new-found cognitive abilities will disappear just as quickly as they showed up, so I'm just trying to enjoy it while it lasts. Luckily, we are staying with family who is taking care of things like cooking to give us a chance to rest.
Unfortunately, my husband does not experience this phenomenon; he's feeling just as crappy as ever, and is quite depressed that we are so close to so many countries that he'd like to visit, yet it feels completely out of reach for him to do much of anything.
I would like to understand why some people experience this and others don't, but I guess that's just one more mystery to add to the mountain of unanswered questions.