With a bit of scheduling and a lot of strategy, you may find you can be more productive working fewer hours.
The traditional workday is not kind to those who do creative work, and it doesn't seem to be getting better. If you work in a field that falls under the wide umbrella of "knowledge work"--anything from writing to programming to complex mathematical problem solving and beyond--you may find yourself in a work environment where 40 hours isn't a ceiling but a starting point.