A letter to graduate students from faculty at some astronomy department has circulated on the interwebs lately. Starts With a Bang has a good take on it.
I'd like to highlight just one thing in that letter:
if you informally canvass the faculty (those people for whose jobs you came here to train), most will tell you that they worked 80-100 hours/week in graduate school. No one told us to work those hours, but we enjoyed what we were doing enough to want to do so. We were almost always at the office, including at night and on weekends.
Working 80-100 hours a week — all nights and all weekends? Bullshit. I've been studying or working at four or five separate places so far, with hundreds of people in total, and I've yet to see anybody, faculty or student, keep that kind of pace on a sustained basis. Yes, you may do a 100-hour week or two when it's crunch time. A hard deadline looms large; you're running a time-intensive experiment; or you have limited-time access to some special equipment. A week or two can occasionally be necessary, just like in many jobs. But nobody keeps up that pace permanently, and nobody can.
You can see the occasional grad student staying late every night, always sitting in the lab if you come in on a weekend. In practice, though, they're rarely actually working; they simply find the lab a comfortable, quiet place for web surfing, chatting or online gaming. They may have intended to work, but in practise they don't get much done. Spending 80 hours a week at work is not the same thing as actually working for all that time.
And that's a general truth: trying to work above a set number of weekly hours — around 40 hours or so - will simply add more time for procrastination, not increase your productivity. We can't keep our concentration up for that long on a sustained basis. If you really want to be effective, stick to around 40 hours of sustained, actual work, then use the rest of the time to recharge.
My guess is that those letter writers well know that. They know how much they can work themselves, and some of them may even feel a bit guilty and ashamed that they don't actually work for as long as everybody claims they do. Also, they're likely somewhat misremembering how much time they actually spent working versus time spent playing, talking, drinking beer and just generally behaving like young people in general. If somebody says "You'll have to work 100 hours a week — we did!", then take that advice with a rather large lump of salt.