Normally I can only do about 4hr/day of productive work, but with my ADD meds I can do 6-8 (big surprise: amphetamines allow you to work more). I do work longer hours, but I don't try to squeeze every ounce of mental energy; I spend the extra time catching up on all of the planning and documentation that doesn't involve intense mental focus. In my middle age I've actually started enjoying the planning and documentation process; to me, it's part of the craft, and what makes the difference between good software and great software. Trying to code for 12 hours straight is counter-productive, but spending an extra four hours tidying up really does add a lot of value.