There's a lot of variation, both due to school and due to department. When I did my PhD, I found the Biosciences department to be rather brutal. Drinking and misery poker (and Magic the Gathering) were the standard pastimes, and long, tedious hours were the norm. On the other hand, the CS department was much more laid back, with much more time put into 'cerebral' work. It's not that the CS students were smarter, I knew a number of brilliant biologists, but the nature of subject matter and the culture. Biology experiments are inherently physical, and can fail or die if not attended to properly, putting a pretty tight time constraint on the experimenter. CS work is much more forgiving, and much more of it (proportionally) happens in the mind. Biology is also a much more grind-centered culture, perhaps in part because it lacks a strong history of internal tool building, while tedious work in CS tends to inspire attempts to build automation and higher level methods. I think some of this is the difficulty (tools in Biology are just harder to make), and some of this is purely culture. I would argue that the somewhat bizarre field of Biophysics is, historically speaking, the tooling culture of Biology that was severed off when Molecular Biology became a field in it's own right and the tools were commoditized, and it's partially this split that's responsible for the culture there.

All of which is to say, grad school is horrible. Sometimes. Sometimes it's awesome, and YMMV.