When's the last time you left work when it was light out? The summertime? Do you justify it by noting daylight savings time? Or, are you working much longer hours than normal and not really taking the full benefit of what a flexible work schedule can offer?
Some jobs can be subject to unfair, tyrannical rules and procedures that make like miserable for pretty much everyone involved. Showing up to the office no later than 8PM Monday through Friday and working late whenever needed isn't exactly my idea of a good work-life balance. I can be counted as somewhat hypocritical in this regard because I do, after all, work much, much, much more that 40 billable hours per week, but I also do my best to make my essential knowledge and work ethic work for me by working alternative hours whenever possible.
If you're going to be on call, or asked to put in lots of extra hours on a regular basis, you should definitely consider asking for a schedule that suites your needs a bit better. Most jobs have the capability to allow their employees to work remotely. Many managers simply don't let their employees work from home because they feel it is taboo and bad for team morale and productivity. Most studies on working from home have proven that theory to be wrong.
Having a healthy work-life balance is essential to your personal and professional progression in life. There's no real reason, aside from an inflexible culture that you cannot have a flexible working schedule that allows you to work remotely or have alternative hours if you're putting in the hard work and time needed to get the job done.
Finding smart, reliable, hard working employees is hard, keeping them happy shouldn't have to be.