Do you know the feeling when it seems like the infernally hot summer has lasted forever and you thought it was never going to end and that you were doomed to be depressed for the rest of your life and then, suddenly, quite out of the blue, there's wind!
The first scent of autumn in the morning air! The sun seems no longer hostile! The world smiles at you! Food tastes better, the day is full of promise, two robins are flying nearby!
There's nothing like fresh air and strong wind in particular. It simply makes everything better. It puts everything into perspective.
So whenever I have a bad mood or feel pessimistic, I tend to go out for a walk. Though I must admit during the summer, if it's hot and not windy, the fresh air effect is not very powerful, so I choose a very late hour to do it when the temperature has dropped or at least the sun won't be up anymore. But this summer has been an exception in weather so I won't judge based on it alone. Usually going out, regardless or the time of day or season, works.
Just to walk. There's no need to do anything special like running or exercising. Walking, with no other agenda than to just wonder around in thought or thoughtless. The fresh air will do it's job by giving your brain more oxygen. You'll notice it has succeeded when the bad mood has lifted or you see a new solution to an issue. Or when you realize your perspective has shifted during the walk, or that whatever was on your mind suddenly seems insignificant.
So here's to autumn and fresh air!
P.S. Neglecting sleep and eating might, in some rare cases, also cause bad moods. Remember to cater to those as well.