Most programming jobs combine creativity with more mundane work. For example, tracking down bugs or answering e-mail questions from co-workers or customers generally doesn't require the amount of creativity that solving a previously unsolved problem does. Neither does figuring out how some open source API works (whoever designed it originally did the heavy lifting for us).
So if we reserve the hard stuff for our hours of peak creativity, we could optimize our creative output while still getting a lot of other useful stuff done in the remaining hours of an eight hour day. Even if my brain is completely fried, I can usually find something to do that pushes my work forward.