A corollary of FortyHourWeek.  

There comes a time in a day of programming at which your productivity and ability to develop decreases, to the point at which you're no longer doing useful work.  Continue beyond that point, and your productivity actually becomes negative:  tired programmers create more problems than they solve.  No matter how good you are, sometimes the most productive thing you can do is JustGoHome.

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No matter how high you have flown into the abstracted meta-realms, do NOT forget to stop and smell the flowers (or listen to the sound of the birds singing).