How do we end the culture of “endless hours at work”?
In the different groups I worked in, their respective cultures were always the result of the behaviour of the leader. So lead by example. In particular, some useful hints from my experience would be:

E-Mails: don't send e-mails in non-office hours, especially not requests for help with something. No e-mails on weekends and late night are a signal which everybody "gets" after a short while. 

Social activities: support group's social life. A group hike to nearby mountains (in office hours on a working day!), or a joint trip to a museum sends a signal that taking time off is an important part of one's life. We used to do it on occasion of having a guest researcher (for a study stay, or just a seminar talk). It's good for group's cohesion and again tells people that rest time is also very useful. 

Personal relationship: care for private lives of your group members. Build relationship with their families. Speak to them about their personal activities, about their vacations, kids, etc. and do not forget to give back and speak about yours. These kinds of discussions send a powerful signal that you perceive time off in a positive way and even encourage it, since you take it as well.

Plan well ahead: clear plans and roadmaps in projects allow people to plan their time off as well. Nothing more annoying than a spurt interfering with one's private life to deliver some report, because the boss didn't care to tell the group ahead.

Time in the office: being in computer science, where I do not need to spend time in a lab and can easily work anywhere, I was always lucky to have bosses who cared for deliverables, rather than for my time in the office. After all, most of my good ideas are born in weird places, such as under shower, while on a bike, walking in woods, and by reading stuff outdoors. Making it clear with the group members that you care first and foremost for deliverables and are flexible regarding the actual time in the office (within reasonable bounds and respecting the local laws and regulations) works well. My experience is also that people tend to deliver better if they are given the power to plan their time and process, rather being forced to mantinels set by their supervisor.

Generally, I found it always very comfortable in groups where having kids was something nice and for what the group leader and members cared. Nothing is worse than a group where work interacts negatively with one's family life and where going home before 6pm to care for kids at home is perceived as something bad.