Look at your sleep patterns. If you notice you are sleeping less (non-intentionally) its usually the easiest way to indicate a mood shift. If you sleep too much it can indicate a depressive phase.

What is too much/too little depends on the individual.

If in doubt talk to your health provider.

Personally I don't think about my happiness/unhappiness too much because feelings are fleeting. I tend to look at thoughts/behaviors. Sleep is something that is easy to track.