What
@halcyondays said. Oftentimes I'll start to spiral downward when I'm tired. Sleep is normally just the elixir I need to get myself back on track.
I'm mostly a glass half full guy. I know it's normal for everyone to have their up and down times.
When I find myself stressed and/or down (they often go together), I find doing some introspection about all I have to be grateful for helps. It helps to put things in perspective and makes me realize whatever I'm beating myself up about is not worth my time.
I also find talking with Hubby or my two besties about that which is on my mind will help me put things in proper perspective.