I'm going to disagree with the other posters. At least in investment banking, the hours are driven by the pace of corporate transactions. Mergers and divestitures are fragile things, and there is a tremendous incentive to get them done as quickly as possible. Investment bankers are at the service of their clients, and when the client wants someone to run the numbers on some idea on Thursday evening so he can have them for a meeting Friday morning, the bankers have to get it done. And that work has to be done by the people who have intimate knowledge of the deal, not some night-shift crew. Companies paying tens of millions of dollars in bankers' fees demand a certain level of service.