In an essay published in The Economist in 1955, British historian Cyril Northcote Parkinson noted how work often “expands so as to fill the time available for its completion.” Writing a few lines on a postcard, he observed, can take an entire day if one has the entire day to write it in. Various factors, from perfectionism to procrastination and laziness to lassitude, contribute to this phenomenon, which has come to be known as Parkinson’s Law. When it applies to why you don’t get more done by working for three hours straight than by working for two, however, Parkinson’s Law may be explained by the rhythmic cycle of your mind.
Your brain swings itself like a pendulum across different states while you sleep. Every ninety minutes or so, you cycle between deep, dreamless sleep, when your mind is motionless but your body is not, and dream-filled REM sleep, when your body is motionless but your mind is not. In the 1960s, American physiologist Nathaniel Kleitman and others speculated that this 90-minute pendulum is also in motion while you are awake, driving you through subtly rising and falling states of alertness. Kleitman called this cycle the basic rest-activity cycle, or BRAC, and surmised that the cycle is barely perceptible in a healthy person because it is easy to override.
Working on something continuously without a break for more than about ninety minutes feels tiring for most of us. When people resume intense mental work after taking a refreshing break, they stay alert and focused for about ninety minutes before returning to the same state of fatigue as before they took the break. As mental tiredness kicks in, your mind drifts and your performance starts to sink, but your brain tries to compensate for its waning performance. A Florida-based research team has identified at least one brain region that may be involved in this compensatory mechanism, it swings into action when other brain regions start to slow down.
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