If you have not yet listened to Kevin Wilde's interview with David, I suggest you do. Kevin made many great points/observations regarding the value of GTD. Among them was his insight into managing your moods. I see my moods as having different types - and levels - of energy.
Where this has helped me as well is in the development of my "rhythm of GTD." Since GTD has the 5 distinct phases of workflow, I am aware of when my mood (somewhat unpredictable for me) may match up to, say Processing and Organizing, or doing the weekly review, or Doing some creative task like writing or editing some part of my GTD presentation for an upcoming event.
For instance, when I return to my home office after traveling there is usually a mood/energy present that best matches up with Processing and Organizing.
On weekends, there tends to me a more creative mood/energy available to me so I will often write or read.
I believe there is a rhythm to life, and we are more relaxed when we are in that rhythm. For me, it is a beautiful thing.