Brain Rules (Updated and Expanded). John Medina

experiences. This is surprisingly similar to the buzzing of a wristwatch’s internal quartz crystal. An area of the brain called the suprachiasmatic nucleus appears to contain just such a timing device. Of course, we have not been characterizing these pulsing rhythms as a benign wristwatch. We have been characterizing them as a war. One of Kleitman and Dement’s greatest contributions was to show that this nearly automatic rhythm occurs as a result of the continuous conflict between two opposing forces.

       Are you a lark, owl, or hummingbird?

      Each of us wages this war on a slightly different schedule. The late advice columnist Ann Landers apparently would take her phone off the hook between 1:00 a.m. and 10:00 a.m. Why? This was the time she normally slept. “No one’s going to call me,” she said, “until I’m ready.” The cartoonist Scott Adams, creator of the comic strip Dilbert, never would think of starting his day at 10:00 a.m. “I’m quite tuned into my rhythms,” he told the authors of The Body Clock Guide to Better Health. “I never try to do any creating past noon. … I do the strip from 6:00 to 7:00 a.m.” Here we have two creative and well-accomplished professionals, one who starts working just as the other’s workday is finished.

      About one in 10 of us is like Dilbert’s Adams. The scientific literature calls such people larks (more palatable than the proper term, “early chronotype”). In general, larks report being most alert around noon and feel most productive at work a few hours before they eat lunch. They don’t need an alarm clock, because they invariably get up before the alarm rings—often before 6:00 a.m. Larks cheerfully report their favorite mealtime as breakfast and generally consume much less coffee than non-larks. Getting increasingly drowsy in the early evening, most larks go to bed (or want to go to bed) around 9:00 p.m.

      Larks are incomprehensible to the one in 10 humans who lie at the other extreme of the sleep spectrum: “late chronotypes,” or owls. In general, owls report being most alert around 6:00 p.m., experiencing their most productive work times in the late evening. They rarely want to go to bed before 3:00 a.m. Owls invariably need an alarm clock to get them up in the morning, with extreme owls requiring multiple alarms to ensure arousal. Indeed, if owls had their druthers, most would not wake up much before 10:00 a.m. Not surprisingly, late chronotypes report their favorite mealtime as dinner, and they would drink gallons of coffee all day long to prop themselves up at work if given the opportunity. If it sounds to you as though owls do not sleep as well as larks in American society, you are right on the money. Indeed, late chronotypes usually accumulate a massive “sleep debt” as they go through life.

      Whether lark or owl, researchers think these patterns are detectable in early childhood and burned into genes that govern our sleep/wake cycle. At least one study shows that if Mom or Dad is a lark, half of their kids will be, too. Larks and owls, though, cover only about 20 percent of the population. The rest of us are called hummingbirds. True to the idea of a continuum, some hummingbirds are more owlish, some are more larkish, and some are in between.

       Nappin’ in the free world

      It must have taken some getting used to, if you were a staffer in the socially conservative early 1960s. Lyndon Baines Johnson, 36th president of the United States and leader of the free world, routinely closed the door to his office in the midafternoon and put on his pajamas. He then proceeded to take a 30-minute nap. Rising refreshed, he would then resume his role as commander in chief. Such presidential behavior might seem downright weird. But if you asked a sleep researcher like Dement, his response might surprise you: It was LBJ who was acting normally. The rest of us, who refuse to bring our pajamas to work, are the abnormal ones.

      LBJ was responding to something experienced by nearly everyone on the planet. It goes by many names—the midday yawn, the post-lunch dip, the afternoon “sleepies.” We’ll call it the nap zone, a period of time in the midafternoon when we experience transient sleepiness. It can be nearly impossible to get anything done during this time, and if you attempt to push through, which is what most of us do, you can spend much of your afternoon fighting a gnawing tiredness. It’s a fight because the brain really wants to take a nap and doesn’t care what its owner is doing. The concept of “siesta,” institutionalized in many other cultures, may have come as an explicit reaction to the nap zone.

      At first, scientists didn’t believe the nap zone existed except as an artifact of sleep deprivation. That has changed. We now know that some people feel it more intensely than others. We know it is not related to a big lunch (although a big lunch, especially one loaded with carbs, can greatly increase its intensity). We also know that when you chart the process S curve and process C curve, you can see that they flatline in the same place—in the afternoon. The biochemical battle reaches a climactic stalemate. An equal tension now exists between the two drives, which extracts a great deal of energy to maintain. Some researchers, though not all, think this equanimity in tension drives the need to nap. Some think that a long sleep at night and a short midday nap represent default human sleep behavior, that it is part of our evolutionary history.

      Regardless of the cause, the nap zone matters, because our brains don’t work as well during it. If you are a public speaker, you already know it is darn near fatal to give a talk in the midafternoon. The nap zone also is literally fatal: More traffic accidents occur during it than at any other time of the day.

      If you embrace the need to nap rather than pushing through, as LBJ found, your brain will work better afterward. One NASA study showed that a 26-minute nap reduced a flight crew’s lapses in awareness by 34 percent, compared to a control group who didn’t nap. Nappers also saw a 16 percent improvement in reaction times. And their performance stayed consistent throughout the day rather than dropping off at the end of a flight or at night. (The flight crew was given a 40-minute break, it took about six minutes for people to fall asleep, and the average nap lasted 26 minutes.) Another study showed that a 45-minute nap produces a similar boost in cognitive performance, a boost lasting more than six hours. Also, napping for 30 minutes before pulling an all-nighter keeps your mind sharper in the wee hours.

      What happens if we don’t get enough sleep

      Given our understanding of how and when we sleep, you might expect that scientists would have an answer to the question of how much sleep we need. Indeed, they do. The answer is: We don’t know. You did not read that wrong. After all of these centuries of experience with sleep, we still don’t know how much of the stuff people actually need. Generalizations don’t work. When you dig into the data on humans, what you find is not remarkable uniformity but remarkable individuality. To make matters worse, sleep schedules are unbelievably dynamic. They change with age. They change with gender. They change depending upon whether or not you are pregnant, and whether or not you are going through puberty. One must take into account so many variables that it almost feels as though we’ve asked the wrong question.

      So let’s invert the query. How much sleep don’t you need? In other words, what are the numbers that disrupt normal function?

       Sleep loss = brain drain

      One study showed that a highly successful student can be set up for a precipitous academic fall just by getting less than seven hours of sleep a night. Take an A student used to scoring in the top 10 percent of virtually anything she does. If she gets just under seven hours of sleep on weekdays, and about 40 minutes more on weekends, her scores will begin to match the scores of the bottom 9 percent of individuals who are getting enough sleep. Cumulative losses during the week add up to cumulative deficits during the weekend—and, if not paid for, that sleep debt will be carried into the next week.

      Another study followed soldiers responsible for operating complex military hardware. One night’s loss of sleep resulted in about a 30 percent loss in overall cognitive skill, with a subsequent drop in performance. Bump that to two nights of sleep loss, and the loss in cognitive skill doubles to 60 percent.

      Other studies showed that when sleep was restricted to six hours or less per night for just five nights, cognitive performance matched that of a person suffering from 48 hours of continual sleep deprivation.

      What do these data tell us? That some people need at least seven hours of sleep a night. And that some people need at least six hours of sleep