[wellboy]

Humans work in cycles.

So, you already had the right hunch.

You don't really have a good direction on a random day and can't really get into the zone? Fix some small bugs for 2h and then call it quits, go to the gym, drink beer with your friends.

You got that one feature that you know exactly how you build it and you're on fire for it? Knock yourself out in a coding session of 2 x 20h and take 3 days off afterwards. Much bette than forcing yourself to quit at 6pm even though you're burning to do it.

Productivity means taking advantage of the human cycle, which is comprised of your mood, hormones, the characteristic of your problem and many more factors that you can't really control.