Merlin Mann wrote a recent post on Ultradian Rhythms–cyclic biological occurrences in the human body. Many systems in the human body apparently work in 90 minute cycles. This is an interesting concept to ponder. Apparently, for maximum performance one should take a physical and mental break every 90 minutes. Concentration begins to suffer after 90 minutes, and these breaks supposedly reduce stress.
Interestingly enough, this is the exact point at which a symphony orchestra takes a rehearsal break. I will pay attention to my concentration cycles during the day for the next while and see if I can detect 90 minutes cycles in my daily life.