Hmm. I wonder how other execs feel. It was amazing how often, when I'd done a diagnostic in a breakthrough-oriented project, that virtually all of the execs in a top management team (approached one-on-one) would lament an aspect of the culture they didn't like, but felt powerless to change it. In effect (in your case) "I don't like crazy hours, but I have to put them in because everyone else does." If that's close to the case, culture change is possible, but I've seen it only when there's a purpose and vision that the organization can become committed to...and, as a side effect, will generate a shift in how the organization operates.
I know how to generate that as part of a coaching/consulting process...not quite sure how that helps you, though.
Well, maybe a place to start is to find out how other execs feel about the long hours. Ask them what purpose they serve, and how that links to what the organization values at a higher level. And also if there are any unintended consequences. See if you can identify something even more important that the long hours inhibit...and what would have to happen for it be ok to shift to an alternative strategy that supports both this new value, as well as whatever "legitimate" benefit the long hours had in the first question you asked. And see if there are any experiments you could identify that would allow you to play with a different approach.
Sounds like the CEO would need to be one of the people interviewed fairly early. If he's not onboard, unlikely others will be.