Well, first, the Sun cannot truly be his power source. Both implied by the magnitude of his feats plus explicitly stated in various books, he possesses far more power than a single yellow star. He's blasted a copy of Hal/GL with heat vision on the order of a hundred suns. He's smashed an Intergang robot while feeling the power of a million suns swell up in his body (and not the first time according to that story).
I assumed that statements like this are colorful hyperbole; Shakespeare once said a character hates another "with the heat of a thousand suns." Which does
not mean they actually experience the temperature of a thousand G-type stars!
One possible instance that may contradict this assertion, Superboy was able to be overpowered by the Sun-Eater during the Legion story arc, because supposedly, "it has the power of hundreds of yellow suns!" And statements were made such as "even Superboy, who gets his power from a yellow sun, is like a GNAT to IT!"
However, this does raise an interesting question, which is, is it possible that Superman uses an alternate power source apart from earth's sun? If so, what sort of power would it be, and what evidence could be pointed to for this phenomenon?
As a personal aside, I generally don't like this sort of theory that implies that what we know about a superhero (e.g. Superman getting his powers from something OTHER than Earth's yellow sun) is totally incorrect. For instance, a friend of mine argued on one occasion that scientifically speaking, it is likely the Flash changes into some type of energy when he moves at superspeed, not unlike the Negative Man or Monica Rambeau. While this sounds interesting, the fact is, that's NOT how the Flash's powers have been demonstrated or explained as working. The Flash runs really fast; he doesn't turn himself into particles and back again.
Second, fatigue poisons only develop when one incurs an oxygen debt from using glucose/energy stores up faster than we can take in oxygen to process it completely into carbon dioxide and water. Assuming his body is using Kryptonian matter analogues of these substances, he would only exert himself at these levels when facing someone of Kryptonian power-levels. Also, an aspect of his super-metabolism, invulnerability, and perhaps super-speed is that should he develop such toxins, he would metabolize them far faster than we would so the recovery period would be hardly noticeable to us should it ever happen to him -- which should be extremely rarely if at all.
Hmm, that makes sense if you assume Superman's inexhaustability is a threshold that is above human levels, instead of an "all or nothing" phenomenon (he either never gets tired or he does). Though Superman's super-metabolism is an interesting explanation for why fatigue never develops.
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