How would Batman get enough rest?
The difficulty for Batman is he's going to be trying to sleep during the day. He's going to be really tired, actually, unless he can shift himself over to just being up at night. If he were just a nocturnal guy, he would actually be a lot healthier and have a lot better sleep than if he were doing what he does now, which is getting some light here and there. That's going to mess up his sleep patterns and duration of sleep.
Wouldn't fighting Gotham's thugs every night take its toll?
The biggest unreal part of the way Batman's portrayed is the nature of his injuries. Most of the time, in the comics and in the movies, even when he wins, he usually winds up taking a pretty good beating. There's a real failure to show the cumulative effect of that. The next day he's shown out there doing the same thing again. He'd likely be quite tired and injured.
Is there any indication in the comics of how long Batman's career lasts?
The comics are really vague on this, of course. In Frank Miller's The Dark Knight Returns, he deliberately shows an aging Batman coming back after he's retired, and he highlights him being tired and weaker. Somewhere around age 50 to 55, he should probably retire. His performance is going down. He's always facing younger adversaries. That is well at the end of when he's going to be able to defend himself and be able to not have to deal that lethal force. This was actually shown in an animated series called Batman Beyond.
Oh right. It's the future; Batman is old and he trains a kid to replace him.
You're familiar with that one? What we learn is that Batman, when he was older but before he retired, actually picked up a gun against a thug because he had to. His skills had let him down so that he wasn't able to defend himself without harming another person. So that's when he decided to retire.
How would all those beat-downs have affected his longevity?
Keeping in mind that being Batman means never losing: If you look at consecutive events where professional fighters have to defend their titles—Muhammad Ali, George Foreman, Ultimate Fighters—the longest period you're going to find is about two to three years. That dovetails nicely with the average career for NFL running backs. It's about three years. (That's the statistic I got from the NFL Players Association Web site.) The point is, it's not very long. It's really hard to become Batman in the first place, and it's hard to maintain it when you get there.
There's research suggesting that concussions might cause depression in NFL players. Could that be one reason why the Dark Knight is so brooding?
I went through a lot of comics and graphic novels and I only found a couple of examples where some of those blows to Batman's head had the effect of something like a concussion. Whereas in reality, that would be a very likely outcome. He's able to offset some of the physical damage to his head because of the cowl—it works a bit like a helmet. But these things would definitely add up. Since they don't admit that he has concussions, you can't really ascribe repeated concussions as the reason why he's brooding.