Nearly all cancer that occurs in the gut is found in the large intestine and almost never in the small intestine. Although no one knows why for sure, many researchers think that it is because of the abundance of bacteria in the former. Professor Hans Clevers of the University of Utrecht in the Netherlands thinks it is related to diet. “Mice get cancer in the small intestine but not in the colon,” he says. “But if you give them a Western-style diet, that reverses. It is the same for Japanese people when they move to the West and adopt a Western lifestyle. They get less stomach cancer, but more colon cancer.”

  * * *

  —

  The first person in modern times to take a close scientific interest in stools was Theodor Escherich (1857–1911), a young pediatric researcher in Munich who began microscopically examining babies’ stools in the late nineteenth century. He found nineteen different kinds of microorganisms there, which was considerably more than he expected to find because the only obvious sources of input were their mothers’ milk and the air they breathed. The most abundant of these is called Escherichia coli in his honor. (Escherich himself called it Bacteria coli commune.)

  E. coli has become the most studied microbe on the planet. It has spawned literally hundreds of thousands of papers, according to Carl Zimmer, who has written a fascinating book, Microcosm, on this single extraordinary bacillus. Two strains of E. coli have more genetic variability than all the mammals on Earth put together. Poor Theodor Escherich never knew any of this. E. coli wasn’t named for him until 1918, seven years after his death, and the name wasn’t officially accepted until 1958.

  Finally, a word or two about flatus, the well-bred term for a fart. Flatus consists primarily of carbon dioxide (up to 50 percent), hydrogen (up to 40 percent), and nitrogen (up to 20 percent), though the exact proportions will vary from person to person and indeed from day to day. About a third of people produce methane, a notorious greenhouse gas, while two-thirds produce none at all. (Or at least none on the occasions on which they have been tested; flatus testing is not the most exacting of disciplines.) The smell of a fart is composed largely of hydrogen sulfide, even though hydrogen sulfide accounts for only about one to three parts per million of what is expelled. Hydrogen sulfide in concentrated form—as in sewage gas—can be highly lethal, but why we are so sensitive to it in trace exposures is a question science has yet to answer. Curiously, we don’t smell it at all when it rises to lethal levels. As Mary Roach put it in her splendid study of all things alimentary, Gulp, “The olfactory nerves become paralyzed.”

  All the gases of flatus can make a pretty explosive combination, as was tragically demonstrated in Nancy, France, in 1978 when surgeons stuck an electrically heated wire up the rectum of a sixty-nine-year-old man to cauterize a polyp and caused an explosion that literally tore the patient apart. According to the journal Gastroenterology, this was just one of “many recorded examples of explosion of colonic gas during anal surgery.” Nowadays, most patients undergo laparoscopic, or keyhole, surgery, which involves being insufflated with—pumped full of—carbon dioxide, and that not only reduces discomfort and scarring but eliminates the risk of explosive mishaps.

  *1 E. coli is a strange organism in that most strains do us no harm and some are positively beneficial—so long as they don’t end up in the wrong place. E. coli in your colon, for instance, produces vitamin K for you—and most welcome that is. We are talking here about strains of E. coli that hurt you or end up where they shouldn’t be.

  *2 St. Martin lived for a time in Cavendish, Vermont, site of the accident that drove an iron bar through the skull of another hapless laborer, Phineas Gage, and also the birthplace of Nettie Stevens, discoverer of the Y chromosome. None of the three were in Cavendish at the same time, however.

  16 SLEEP

  O sleep, O gentle sleep,

  Nature’s soft nurse.

  —WILLIAM SHAKESPEARE, HENRY IV, PART 2

  I

  SLEEPING IS THE most mysterious thing we do. We know that it is vital; we just don’t know exactly why. We can’t say with any certainty what sleep is for, what is the right amount for maximum health and happiness, or why some people fall into it with ease while others struggle perpetually to attain it. We lose a third of our lives to it. I am sixty-six years old as I write this. I have in effect slept through the whole of the twenty-first century.

  There isn’t any part of the body that does not benefit from sleep or suffer from its absence. If you are deprived of it for long enough, you will die—though what exactly it is that kills you from lack of sleep is a mystery, too. In 1989, in an experiment unlikely to be repeated on grounds of cruelty, researchers from the University of Chicago kept ten rats awake until they died and discovered that it took between eleven and thirty-two days for exhaustion to fatally overcome them. Postmortems showed the rats had no abnormalities that could explain their deaths. Their bodies just gave up on them.

  Sleep has been tied to a great many biological processes—consolidating memories, restoring hormonal balance, emptying the brain of accumulated neurotoxins, resetting the immune system. People with early signs of hypertension who slept for one hour more per night than previously showed a significant improvement in their blood pressure readings. It would seem to be, in short, a kind of nightly tune-up for the body. As Professor Loren Frank of the University of California at San Francisco told the journal Nature in 2013, “The story that everyone tells is that sleep is important for transferring memories to the rest of the brain. But the problem is there’s basically no direct evidence for this idea.” But why we should be required to so utterly give up consciousness for this is a question yet to be answered. It isn’t just that we are disengaged from the outside world when slumbering, but for much of the time are actually paralyzed.

  Sleep is clearly about more than just resting. One curious fact is that animals that are hibernating also have periods of sleep. It comes as a surprise to most of us, but hibernation and sleep are not the same thing at all, at least not from a neurological and metabolic perspective. Hibernating is more like being concussed or anesthetized: the subject is unconscious but not actually asleep. So a hibernating animal needs to get a few hours of conventional sleep each day within the larger unconsciousness. A further surprise to most of us is that bears, the most famous of wintry slumberers, don’t actually hibernate. Real hibernation involves profound unconsciousness and a dramatic fall in body temperature—often to around 32 degrees Fahrenheit. By this definition, bears don’t hibernate, because their body temperature stays near normal and they are easily roused. Their winter slumbers are more accurately called a state of torpor.

  Whatever sleep gives us, it is more than just a period of recuperative inactivity. Something must make us crave it deeply to leave ourselves so vulnerable to attack by brigands or predators, yet as far as can be told, sleep does nothing for us that couldn’t equally be done while we were awake but resting. We also do not know why we pass much of the night experiencing the surreal and often unsettling hallucinations that we call dreams. Being chased by zombies or finding yourself unaccountably naked at a bus stop doesn’t seem, on the face of it, a terribly restorative way to while away the hours of darkness.

  And yet it is universally assumed that sleep must answer some deep elemental need. As the eminent sleep researcher Allan Rechtschaffen observed many years ago, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.” Nonetheless, as far as we know, all sleep does is (in the word of another researcher) “make us fit to be awake.”

  All animals seem to sleep. Even quite simple creatures like nematodes and fruit flies have periods of quiescence. The amount of sleep needed varies markedly among animals. Elephants and horses get by on just two or three hours a night. Why they need so little is unknown. Most other mammals require a great deal more. The animal that used to be thought the mammalian sleep champion, the three-toed sloth, is still often said to sleep for up to twenty hours a day, but that number came from studying captive sloths, who have no predators and not a lot to do. Wild sloths slumber for more like ten hours a day—not a huge amount more than we do. Extraordinarily, some birds and marine mammals are able to switch off one half of their brain at a time, so that one half remains alert while the other is snoozing.

  * * *

  —

  Our modern understanding of sleep may be said to date from a December night in 1951 when a young sleep researcher at the University of Chicago named Eugene Aserinsky tried out a machine for measuring brain waves that his lab had acquired. Aserinsky’s volunteer subject for the first night’s test was his eight-year-old son, Armond.

  Ninety minutes after young Armond had settled down into what was normally a peaceful night’s sleep, Aserinsky was surprised to see the monitor’s unspooling graph paper jerk to life and begin the kinds of jagged tracings associated with an active, wakeful mind. But when Aserinsky went next door, he found Armond still fast asleep. His eyes, however, were moving visibly beneath his lids. Aserinsky had just discovered rapid eye movement sleep, the most interesting and mysterious of the multiple phases of our nightly sleep cycle. Aserinsky didn’t exactly rush the news into print. Almost two years passed before a small report on the discovery appeared in the journal Science.*1

  * * *

  —

  We now know that a normal night’s sleep consists of a series of cycles, each involving four or five phases (depending on whose categorization methods you favor). First comes the business of relinquishing consciousness, which for most of us takes between five and fifteen minutes to achieve fully. This is followed by a period in which we slumber lightly but restoratively, as in a nap, for about twenty minutes. Sleep is so shallow in these first two stages that you may be asleep but think you are awake. Then comes a deeper sleep, lasting about an hour, from which it is much harder to rouse a sleeper. (Some authorities divide this period into two stages, giving the sleep cycle five distinct periods rather than four.) Finally comes the rapid eye movement (or REM) phase, when we do most of our dreaming.

  During the REM part of the cycle, the sleeper becomes mostly paralyzed, but the eyes dart about beneath closed lids as if witnessing some urgent melodrama, and the brain grows as lively as at any time during wakefulness. In fact, some parts of the forebrain are livelier during REM sleep than when we are fully conscious and moving around.

  Why the eyes move during REM sleep is uncertain. One obvious idea is that we are “watching” our dreams. Not all of you is paralyzed during the REM phase. Your heart and lungs continue to function, for obvious reasons, and clearly your eyes are free to swivel, but the muscles that control bodily movement are all restrained. The explanation most often proposed is that immobilization stops us from harming ourselves by thrashing about or trying to flee from attack when caught up in a bad dream. A very few people suffer from a condition called REM sleep behavior disorder in which the limbs don’t become paralyzed, and they do indeed sometimes hurt themselves or their partners with their thrashing. For others, paralysis doesn’t immediately abate upon awaking and the victim finds himself awake but unable to move—a deeply unnerving experience, it seems, but one that mercifully tends to last only for a few moments.

  REM sleep accounts for up to two hours of every night’s sleep, roughly a quarter of the total. As the night passes, the periods of REM sleep tend to lengthen, so that your most dreamy spells are usually in the final hours before waking.

  The cycles of sleep are repeated four or five times a night. Each cycle lasts about ninety minutes, but can vary. REM sleep is seemingly important for development. Newborn babies spend at least 50 percent of their sleep time (which is most of their time anyway) in the REM phase. For fetuses it may be as much as 80 percent. For a long time, it was thought that we did all our dreaming during REM sleep, but a 2017 study at the University of Wisconsin found that 71 percent of people dreamed during non-REM sleep (as compared with 95 percent during REM sleep). Most men have erections during REM sleep. Women likewise experience increased blood flow to the genitals. No one knows why, but it seems not to be overtly associated with erotic impulses. Typically, a man will be erect for two hours or so a night.

  We are more restless at night than most of us realize. The average person turns over or significantly changes position between thirty and forty times in the course of a night. We also wake up far more than you might think. Arousals and brief awakenings in the night can add up to thirty minutes without being noticed. On a visit to a sleep clinic for his 1995 book, Night, the writer A. Alvarez thought he had experienced an unbroken night’s sleep but discovered when his chart was reviewed in the morning that he had woken up twenty-three times. He also had five dreaming periods of which he had no recollection.

  As well as normal overnight sleep, we also commonly indulge in snatches of wakeful-hours sleep in a state known as hypnagogia, a netherworld between waking and unconsciousness, often without being aware of it. Alarmingly, when a dozen airline pilots on long-haul flights were studied by sleep scientists, almost all were found to have been asleep, or all but asleep, at various times during the flight without realizing it.

  The relationship between the sleeping person and the outside world is often a curious one. Most of us have experienced that abrupt feeling of falling while asleep known as a hypnic or myoclonic jerk. No one knows why we have this sensation. One theory is that it goes back to the days when we slept in trees and had to take care not to fall off. The jerk may be a kind of fire drill. That may seem far-fetched, but it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances). Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers. Studies at Oxford University, related by Paul Martin in his book Counting Sheep, found that EEG readings for test subjects twitched whenever their own names were read aloud as they slept but didn’t react when other, unknown names were recited. Tests have also shown that people are pretty good at waking themselves at a predetermined time without an alarm clock, which means that some part of the sleeping mind must be tracking the real world outside the skull.

  Dreaming may simply be a by-product of this nightly cerebral housecleaning. As the brain clears wastes and consolidates memories, neural circuits fire randomly, briefly throwing up fragmentary images, a bit like someone jumping between television channels when looking for something to watch. Confronted with this incoherent flow of memories, anxieties, fantasies, suppressed emotions, and the like, the brain possibly tries to make a sensible narrative out of it all, or possibly, because it is itself resting, doesn’t try at all, and just lets the incoherent pulses flow past. That may explain why we generally don’t remember dreams much despite their intensity—because they are not actually meaningful or important.

  II

  IN 1999, AFTER ten years of careful work, a researcher at Imperial College in London named Russell Foster proved something that seemed so unlikely that most people refused to believe it. Foster found that our eyes contain a third photoreceptor cell type in addition to the well-known rods and cones. These additional receptors, known as photosensitive retinal ganglion cells, have nothing to do with vision but exist simply to detect brightness—to know when it is daytime and when night. They pass this information on to two tiny bundles of neurons within the brain, roughly the size of a pinhead, embedded in the hypothalamus and known as suprachiasmatic nuclei. These two bundles (one in each hemisphere) control our circadian rhythms. They are the body’s alarm clocks. They tell us when to rise and shine and when to call it a day.

  All that may seem eminently sensible and good to know, but when Foster announced his discovery, it caused the most enormous outcry in the ophthalmological world. Almost no one could believe that such a fundamental thing as an ocular cell type could have been overlooked for so long. One member of an audience shouted, “Bullshit!” at one of Foster’s presentations and stalked out.

  “They struggled to accept that something they had been studying for 150 years—namely, the human eye—had a type of cell whose function they had completely overlooked,” he says. In fact, Foster was right and has since been completely vindicated. “They’re much more gracious about it now,” he jokes. Today Foster is professor of circadian neuroscience and head of the Nuffield Laboratory of Ophthalmology at Oxford University.

  “What’s really interesting about these third receptors,” Foster told me when we met in his office at Brasenose College, just off the High Street, “is that they function completely independently of sight. As an experiment, we asked a lady who was completely blind—she had lost her rods and cones as a result of a genetic disease—to tell us when she thought the lights in the room were switched on or off. She told us not to be ridiculous because she couldn’t see anything, but we asked her to try anyway. It turned out she was right every time. Even though she had no vision—no way of ‘seeing’ the light—her brain detected it with perfect fidelity at a subliminal level. She was astonished. We all were.”