In the 1950s, the researchers William Dement and Nathaniel Kleitman discovered that we sleep in cycles of roughly 90 minutes, moving from light to deep sleep and back out again. They named this pattern the Basic-Rest Activity Cycle or BRAC. A decade later, Professor Kleitman discovered that this cycle recapitulates itself during our waking lives. The difference is that during the day we move from a state of alertness progressively into physiological fatigue approximately every 90 minutes. Our bodies regularly tell us to take a break, but we often override these signals and instead stoke ourselves up with caffeine, sugar and our own emergency reserves — the stress hormones adrenaline, noradrenaline and cortisol. Working in 90-minute intervals turns out to be a prescription for maximizing productivity. Professor K. Anders Ericsson and his colleagues at Florida State University have studied elite performers, including musicians, athletes, actors and chess players. In each of these fields, Dr. Ericsson found that the best performers typically practice in uninterrupted sessions that last no more than 90 minutes. They begin in the morning, take a break between sessions, and rarely work for more than four and a half hours in any given day.