Whenever I come across some words of wisdom that I agree with, I'd better put them down here so that I will remember. These are taken from Scott Adams' blog :

Sometimes you think you are unhappy, but you are really tired.

And here is a comment that appeared in the Comments section of the blog:

Being a Doctor myself, I always prescribe "diet and exercise" to my depressed patients. They invariably tell me that they don't come to their doctors to get an advice they could get from their mother. So I prescribe them some pills.

It is true that exercise increases your energy. You develop muscles, increase pulmonar capacity, increase oxygen absorbsion in your blood. All good for you.

Your brain, by the way, will happily use more oxygen.

Also, physical exercise "consumes" stress hormones (notably adrenalin) which is a factor in anxiety and depression (adrenalin is that factor). Excercise also promote dopamine production in your brain (the reward "I'm happy") mechanism.

So yes, your theory is pretty accurate. Tired => Inactive => more tired => unhappy. It's all linked.

Now get out of your chair and give me 10.

Now I have an eloquent excuse to go out and cycle more!