He draws his powers from the Sun. He accumilates solar reserves, which enable him to do the things he does. Hence there needing to be a 'Yellow Sun' present for Superman to have them. His solar reserves are always increasing, but as he takes a beating he has to use them.
As a battle draws out, one such as Superman vs Doomsday, if he's beaten to such an extent I presume his solar reserves are depleted. Therefore, his powers gone. I.e Invulnerability.
However, I'm not aware to the extent of how long this may be, or if this is exactly how it works. There may need to be other factors to prevent him drawing from the Sun, I'm not entirely sure. But it's the jist of it.
He's basically like a solar powered battery.
Edit: I agree. I was never a massive fan of Superman. I dabbled in a few things, but nothing major. It was only until I started watching Smallville he was shown in an entirely different light. Shifted the boyscout image. Saw some powers in action. Good stuff.
Since then I've worked my way around message boards learning more and more about Superman, his powers, and other various comic book characters.
Nah. I'm no nerd. I've recently taken an interest in comics, I enjoy debating, so when they come together I'm posting on KMC forums. Speak for yourself, people stereotype nerds as guys who are smart, enjoy computers, etc. But that day is done, bitchez, and a huge range of people access computers regulary now.
Also, I might add, labelling someone is so lame. It doesn't matter what path you take in life, as long as you're satisfied with it. A lack of self-esteem is what makes these idiots critisize others who are different. And by different, I don't mean in a bad way, or good way. What's considered the society 'norm' is a personal judgement, not a fact.