Everyone goes through alternating periods of high and low mental acuity. There are days when I work on personal projects for well over 8 hours, but the time is always divided into multiple sessions.

The ability of a factory worker to think analytically is irrelevant, he’s either cranking widgets or he isn’t.

The obvious solution to this problem is planning around the mental energy cycle by breaking the work day into multiple segments.

It doesn’t make sense because we lose the ability to concentrate effectively within a few hours.

Nobody does their best work 5:30 in the afternoon after they’ve been sucking down coffee all day to stay awake.

For different people the peaks and valleys will vary, but overall I’d estimate only 3-4 hours a day could be classified as highly productive.

The low ratio of highly productive hours to total hours worked is the result of the continuous 8 hour work day.