Norman Vincent Peale:

You only lose energy when life becomes dull in your mind.  Your mind gets bored and therefore tired doing nothing.  You don't have to be tired.  Get interested in something.  Get absolutely enthralled in something.  Throw yourself into it with abandon.  Get out of yourself.  Be somebody.  Do something.  Don't sit around moaning about things, reading the papers, and saying, "Why don't they do something?"  The man who is out doing something isn't tired.  If you're not getting into good causes, no wonder you're tired.  You're disintegrating.  You're deteriorating.  You're dying on the vine.  The more you lose yourself in something, the more energy you will have.