This all comes down to our ultradian rhythms. Like circadian rhythms, ultradian rhythms make up a sort of body clock that cycles regularly throughout our lives. But while circadian rhythms keep track of the 24-hour day/night cycle, ultradian rhythms roll in 90–120 minute cycles; specifically, those of our individual brain-wave frequencies. Most people are familiar with the way we pass through different stages of sleep—these are our ultradian rhythms in action—but you may not realize that you also pass through different stages of waking.

Studies have found that all sorts of things are affected by these cycles, such as dopamine levels, alertness, and especially attention. A 1993 study by psychologist Anders Ericsson found that the best violinists all practiced in three 90-minute chunks with breaks in between. A 1995 study found that people generally maintain their sleep cycles after waking as periodic "sleepiness" cycles. This is why experts such as Buffer CEO Leo Widrich and engagement consultant Tony Schwartz swear by a rest-activity cycle that builds breaks into the work day.