I had heard about stress relief. I only notice if its been a few days of driving in, but I notice I'm more stressed without a consistent bike ride. We're having 40 knot gusts for Friday morning and tomorrow so it'll be a few days that I've wimped out and I'm feeling the out of kilter feeling, less humor.
I think in part not biking allows me to worry about other errands on the way home (biking simplifies what I can do). I let things slide to the morning that I ought to do now (instead I'm reading Bike Forum). I also try harder to get all papers corrected before leaving on the bike, while I don't feel as much pressure if I've got the car.

I did find this blog entry from Boston (contains profanity, and plenty of it) that shows that perhaps biking is not a cure all for stress and anger: http://ridereport.bostonbiker.org/20...midnight-ride/