Also, 8 hours of "work" vs. 8 hours of productivity is quite different. I use pomodoro timers to track my level of productivity and when they add up to 8 hours, my brain is totally done. I'd have to squeeze in a long run to shake myself out of it.
Obsessive tracking of the time is as unhealthy I found. I keep a work log but I never read it. It's mainly for other peoples' benefit.