I think there is something to doing unique things and it taking you out of your comfort zone. For instance in this case I could see hot yoga being vastly different than my typical workouts and that having a positive impact on my moods more so than my typical workout. I think it might be something that would be interesting to study, because while having better cardio and the endorphins that are generated from a good workout do make you feel better I have to imagine just as important for better moods is the fact that you are improving yourself in a way that takes you out of what might be a routine that you associate with depression or negative moods, which I think we all cycle through at points in our lives. Shaking things up seems to be important I guess is what I'm saying.