With that information I should be able to detect the quality of my sleep and how much exercise I get. With some additonal discipline, I might be able to track what foods I eat by cooking for myself. Also just for fun I could also track exposure to light and weather.
Combining that information with some ontology for describing events in my life and my moods I should then be able to empirically determine what kinds of things lead me to be feeling down.
I've always wondered if wanting to do these kinds of things make me seem far too eccentric for people to relate to.