When we are too tired to make good decisions, it can be a good decision to decide to stop making decisions for a while. --RonJeffries on the XpMailingList

This is why even a partner is not enough to help follow this rule effectively, and we fall back on rigid numbers, like FortyHourWeek, as a reminder. 

A better way of stating the rule is WorkWhenYoureFreshStopWhenYoureTired.

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Here's how I know I'm adhering to the spirit of the rule -- I don't look back often and say, "I knew I was being ineffective, but I was too far into the problem to realize it." --KentBeck on XpMailingList
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I find that there are different kinds of work that I do better at various times of the day.  In the morning I am best at analysis.  In the late morning and early afternoon I'm best at the details of programming.  In the late afternoon I'm best at RandomActsOfLearning.

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I think it depends on the person.  Personally I like to enjoy my work and not be forced to do the work.  Usually I like to FrontLoad my time so I can rest up a bit easier later on in theory.  I sporatically get RandomActsOfLearning done duing my FrontLoad period and have to deal with less people at once to boot.  I still adhere to WorkWhenYoureFreshStopWhenYoureTired, I am still young for now and I can have longer runtimes than FortyHourWeek, but having RandomActsOfLearning in the middle helps relieve tiredness and makes you go on longer.-- ArchimedesTrajano