Sara Robinson claims, with a few caveats, that longer work-weeks are counter-productive:
It’s a heresy now (good luck convincing your boss of what I’m about to say), but every hour you work over 40 hours a week is making you less effective and productive over both the short and the long haul. And it may sound weird, but it’s true: the single easiest, fastest thing your company can do to boost its output and profits — starting right now, today — is to get everybody off the 55-hour-a-week treadmill, and back onto a 40-hour footing.
The Dish 