I know many, many people out here in Silicon Valley who, despite being extremely bright in some areas, are ignorant when it comes to being productive. Young, single males with ambition are incredibly likely to pull long hours that hurt them in the long run.
The basic rule of thumb is that If you are pulling >40 hour weeks for more than a couple of months you're being less productive. "Not me", you say, " I was the smartest kid in my class and I have incredible mental endurance!".
Nope, everybody makes bad decisions after working long hours for a few months. For all the coders out there, think about it like this: How many genious decisions have you made at 3am after working for 15 hours? Now think about how many egregious hacks you wrote at 3am to get something out the door. Compare the two.