This is an interesting perspective on energy and surfing, @justin. I tend to be far less calculated when it comes to catching waves.
I do find that on the whole, surfing supplies me a great deal of physical, mental and spiritual energy — as long as I don't take from it more than what feels necessary. If I'm tired and out too long, my chances of paying the price increases the more I keeping waiting for "just one more wave." My wife could get angry later on, too (lol).
Managing fear in larger/trickier waves requires a great deal of mental fortitude and concentration, and an even greater ability to let go without being reckless. The payoff, as I'm sure you know, can be huge: you get a profound boost of energy that lasts for several hours (or more), and a great memory for your mental highlight reel.
Injury aside, I really think the getting-sore-and-needing-to-recover part is a symptom of age A few times this season I've had waves land on me in the worst of ways, and since then my neck has been constantly sore (lately my shoulder, too, and I don't even know where or how that happened!). Ironically, though, it's caused me more pain out of the water than in.
You're 100% right on fatigue and sleep. They're partners in crime. Some days I'm so beat I don't even bother checking the surf, which would only take me a minute to do.
Overall, I pretty much go by feel. I know myself and rhythms well enough to pre-qualify my body for successful surfing. Daily stretching is also paramount.