Also ensures good handoff on projects -- if you work 60h/wk for, say, 8 weeks to ship something, and 40h/wk for 2 weeks before/after to handle turnover, and then are basically AFK for 8 weeks, you'll probably do a pretty good job on handing it off.

(My personal ideal is ~8 days on (12-16h/day), 6 days off, and then a month or two off twice a year.)