Poll: How many hours a day are you effective?

I have always noticed a significant drop in ability after about 6 hours of work and unless urgent I switch to tasks that require far less mental concentration. A good lunch, perhaps with exercise helps recover an extra hour or so at most. Almost invariably, the next morning after a sleep, problems that took considerable effort the previous day now seem clear and straight forward.

So HN, I'm curious how many hours a day you feel you are nearly fully effective in your work.