I haven't really noticed this myself but I haven't been around a lot of Pokemon Go players. I think this has to do more with people being outside more often. Think about it. Over the last 20 years we have transitioned into indoor people. So many people are afraid of the sun these days lol SOURCE
This is because the sun has a natural anti-depressant in it known as vitamin D3! Well, the sun doesn't but the effects of the rays on your skin causes the body to produce it.