Question: I work at a very young start-up company. We have had turn-over rates (mass resignation of staff, thrice) in the past but changes in the system have led to a more a stable employee retention and better technical-skilled employees. We do games, and I don't know with other software fields, but the "[death march](http://arstechnica.com/gaming/2011/05/the-death-march-the-problem-of-crunch-time-in-game-development/)" is quite popular in our field.

My boss believes that the only way for us to catch-up to top companies is to exert more effort, which generally translates to more time working. Hence, we've had a culture of working a lot of hours (we work an average of 65 hours a week, peaking up to 90hours a week).

Our deadlines are either very difficult and sometimes impossible (since the team is pretty young) but our management sets deadlines at the "if people at company X can do this in X days, we can do it too"

I personally believe that raising team efficiency is one of the factors that can help eliminate too much overtime.

I've seen my subordinates/junior staff often not focused enough during the day, but I don't know how to objectively measure this (I've seen questions here relating to KPI's, or key performance indicators).

I'm in a position where I have good relations with my boss (our CEO), the management and the entire team and I believe they're more than willing enough to hear out what I have to say.

How can I approach this in a way that is beneficial for both the staff and the company?

Answer: 
The simple answer is to simply begin working 40 hour work weeks and make sure YOU are efficient for those 40 hours. If you aren't already burned out then the results will become apparent fairly quickly. If you are already burned out then 40 hour weeks probably won't make a difference, so keep working your 60-90 hour work weeks. Keep in mind that if you choose to continue with the long work weeks you'll be looking for a new line of work in the near future and that won't include any careers requiring thinking skills because your body is going to shut down all on its own because you chose not to listen to it.

I really don't understand how people can let themselves be so taken advantage of like your company does to you.
To change from a culture of overtime to efficiency you will have to stop the death marches. Each person has a limit, after that they can actually produce negative work. The density and severity of their errors will mean that that last few hours they worked, the project went backwards. Their mistakes may be obvious , they broke the build. Or it may be subtle, they picked the wrong choice of the size of a field in the database so that next week the tables will have to be rebuilt. 

If you try to tell them that they are inefficient but you still expect 60-90 hours a week they will not have a reason to change. If they can produce more quality code in less hours, but you don't reduce the expected number of hours, they can't help but be demotivated and inefficient.

One way to address the issue runs counter to the arguments in the mythical man month. You need more developers. Yes increasing the number of the people on a project does greatly increase communication issues. But expecting too many hours also is inefficient. Unless you are paying them for every hour of overtime, your costs will rise by adding members to the team. But you have already had 3 incidents of massive numbers of employees quitting.

I know nothing of your company, but a great way to demotivate people is to have management be able to flaunt their free time, hobbies and wealth. Management that leaves every day at 5, and has time for fun filled weekends, without a way to compensate the developers, designers, and artists is doomed to be facing mass exodus #4.