Even though the data around productivity has proved pretty remorseless, humans have found the message hard to accept. It seems so logical that two units of work will produce twice the output. Logical but wrong. The critical measure of work isn't and never should be input but output. What matters isn't how many hours your team puts in, but the quality and quantity of work they produce.

Which is where sleep comes in. Although we might all like to imagine that we can work happily through the night, once again the data's all against us. Lose just one night's sleep and your cognitive capacity is roughly the same as being over the alcohol limit. Yet we regularly hail as heroes the executives who take the red eye, jump into a rental car, and zoom down the highway to the next meeting. Would we, I wonder, be so impressed if they arrived drunk?

The reason sleep is so important is because fatigue isn't simple. When we are tired, our performance doesn't degrade equally. Instead, when you lose a night's sleep, the parietal and occipital lobes in your brain become less active. The parietal lobe integrates information from the senses and is involved in our knowledge of numbers and manipulation of objects. The occipital lobe is involved in visual processing. So the parts of our mind responsible for understanding the world and the data around us start to slow down. This is because the brain is prioritizing the thalamus—the part of your brain responsible for keeping you awake. In evolutionary terms, this makes sense. If you're driven to find food, you need to stay awake and search, not compare recipes.

After 24 hours of sleep deprivation, there is an overall reduction of six percent in glucose reaching the brain. (That's why you crave donuts and candy.) But the loss isn't shared equally; the parietal lobe and the prefrontal cortex lose 12 percent to 14 percent of their glucose. And those are the areas we most need for thinking: for distinguishing between ideas, for social control, and to be able to tell the difference between good and bad.

I am constantly amazed at how many otherwise intelligent people insist on burning the midnight oil, producing important work (reports, essays, etc) late at night. Meanwhile, I get odd looks when I insist on trying to get at least 7.5 hours of sleep (preferably eight) each night.