From her Oct 5, 2007 appearance on Oprah:
"At the end of every day, write down the happiest moment of every day. It's my happiness journal. And it's a way of reminding myself what really makes me happy, and what doesn't. And you know, every day also has its crappiest moment of the day, but I decided not to keep a crappiest moment of the day journal. And learn, and study, and look back and see what is it consistently.
For me, what I've realized, it seems to have a lot to do with photosynthesis, a lot to do with sunlight, you know? So many of it is, like, I'm going to get my mail and the sunlight hits my shoulders and I think, I'm alive, you know? And that's it. I mean, that--there, done, happy moment of the day finished. Now you can go do all your chores."