Sleep

• 1. Introduction

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  Sleep takes up nearly a third of a human life, yet the brain never goes quiet during it. Even at its deepest, sleep stays more reactive than a coma, holding active brain patterns that a sleeper can break through when a loud noise cuts in. Every night the body slips into an anabolic state, quietly restoring the immune, nervous, skeletal, and muscular systems while consciousness dims and the muscles fall still. Researchers think this behavior is hundreds of millions of years old, conserved across animal evolution, and they suspect it began as a way for the brain to wash itself clean of waste. So what is the brain actually doing while a person lies motionless? Why does it cycle between two states so unlike each other that physiologists treat them as separate behaviors? And why has artificial light, glowing from outdoor lamps and the screens of smartphones and televisions, started to bend a rhythm that the sun once set?

• 2. Two Modes Of The Night

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  Non-REM sleep arrives first, and after a transitional period it deepens into slow-wave sleep, when body temperature and heart rate fall and the brain draws less energy. REM sleep, also called paradoxical sleep, takes up a smaller share of the night and runs on the opposite logic. It brings desynchronized, fast brain waves, darting eye movements, a loss of muscle tone that leaves the body nearly paralyzed, and a suspension of homeostasis. Despite its name, rapid eye movement, this mode is far more than moving eyes, and it is the main stage for dreams and nightmares.

  The American Academy of Sleep Medicine splits non-REM into three stages, N1, N2, and N3, the last also called delta sleep or slow-wave sleep. A full cycle normally runs N1, then N2, then N3, back to N2, then into REM, averaging about 90 minutes and repeating four to six times in a good night. REM tends to begin as a sleeper climbs back up toward stage 2 or 1 from deep sleep. Deep stage N3 dominates the early hours, while REM swells across the two cycles just before a person wakes naturally.

• 3. Reading The Sleeping Brain

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  Electroencephalography, or EEG, captures the brain's electrical activity as waves, and the amplitude of those waves at a given frequency maps onto where a person sits in the sleep-wake cycle. Alpha, beta, theta, gamma, and delta waves each carry a distinct frequency and amplitude. Alpha waves appear in a resting state when a person is still fully conscious, eyes perhaps closed and body slowing down. Beta waves take over at attention, with the highest frequencies and lowest amplitude, marking full alertness, while gamma waves show up under intense concentration. Theta waves run through wakefulness and on into stages 1 and 2, and delta waves belong to stages 3 and 4, the deepest sleep.

  Electrooculography, or EOG, tracks eye movements, and electromyography, or EMG, tracks skeletal muscle activity. Collecting these together is called polysomnography, and it can be run in a specialized sleep laboratory. Researchers also fold in simplified electrocardiography for cardiac activity and actigraphy for motor movement. Those same tools reveal how little of the night is spent awake. Sleep studies using EEG found females awake for 0 to 1 percent of nightly sleep and males for 0 to 2 percent, with adult wakefulness climbing in later cycles. One study logged 3 percent awake time in the first ninety-minute cycle, rising through 8, 10, and 12 percent to 13 to 14 percent in the fifth, most of it falling just after REM.

• 4. The Clock And The Debt

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  The suprachiasmatic nucleus, a brain area directly above the optic chiasm, is considered the most important hub for the circadian clock, or Process C, the system that builds an internal day-night rhythm from environmental signals. This nucleus has a direct neural line to the pineal gland, which releases melatonin at night. Lock an entrained human in a bunker with constant light or darkness and the rhythms of body temperature and melatonin keep running, on a period that slightly exceeds 24 hours, a state scientists call free-running. Body temperature oscillates roughly between 36.2 and 37.2 degrees Celsius, cortisol peaks in the awakening hours, and a young adult entrained to the sun reaches a temperature minimum near 6 a.m.

  The longer an organism stays awake, the stronger its need to sleep, a pressure called Process S, or sleep debt. It builds as glycogen is depleted and adenosine accumulates in the forebrain, which disinhibits the ventrolateral preoptic nucleus and quiets the ascending reticular activating system. Sleep deprivation slows brain waves in the frontal cortex, shortens attention, raises anxiety, impairs memory, and sours mood. Adenosine is the chemical marker of that debt, and coffee, tea, and other caffeine sources temporarily block it, prolonging sleep latency and cutting total sleep. Subjectively, humans seem to reach maximum sleepiness about 30 hours after waking.

• 5. How Much, And When

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  Newborns may need up to 18 hours of sleep a day, and that need declines steadily with age. Early in 2015, after a two-year study, the National Sleep Foundation in the US issued revised figures: 14 to 17 hours for newborns aged 0 to 3 months, 12 to 15 hours for infants, dropping through childhood to 7 to 9 hours for adults aged 18 to 64 and 7 to 8 hours for older adults. Sleeping 6 to 7 hours each night correlates with longevity and cardiac health, though many factors may underlie that link.

  Chronotypes capture the familiar split between the early bird and the night owl, shaped by genetics, sex, and habit, and liable to shift across a lifetime. Seven-year-olds wake early more easily than fifteen-year-olds do. Naps fill a different niche, with about one-third of American adults napping daily and an optimal duration of 10 to 20 minutes, since it takes at least 30 minutes to fall into slow-wave sleep. The siesta habit has been linked to 37 percent lower coronary mortality. Genes leave fingerprints too: identical twins share sleep habits while fraternal twins do not, and mice lacking the Dpyd gene slept 78.4 minutes less during the lights-off period than wild-type mice.

• 6. What Sleep Repairs

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  The glymphatic system clears metabolic waste from the brain far faster during sleep than during waking, by ramping up the flow of cerebrospinal fluid. Pulses of hormones drive surges in blood flow that push that fluid through, carrying away metabolites, including amyloid. Anabolic hormones such as growth hormone are secreted preferentially during sleep, and the brain's glycogen stores rise overnight before being spent through waking metabolism. The body restores itself mostly during slow-wave sleep, when body temperature, heart rate, and brain oxygen consumption all fall.

  One researcher summed it up as sleep being of the brain, by the brain and for the brain. The body can run its restorative processes during quiet waking, but the brain actually requires sleep, and the behavior appears across most of the animal kingdom, even in some of the least cognitively advanced creatures. Memory is part of the payoff. Declarative memory improves more during early sleep dominated by slow-wave sleep, while procedural memory firms up during late sleep dominated by REM. Slow-wave sleep is tied to hippocampal replays of encoded patterns, and the active system consolidation hypothesis holds that repeated reactivations during NREM gradually fold new declarative memories into the neocortex.

• 7. The World Of Dreams

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  Dreams feel logical and realistic to the dreamer while they unfold, even as their bizarre and surreal qualities only become obvious after waking. They stitch together people, situations, and objects that would never normally meet, and they fade fast from memory once a person wakes. Some keep a dream journal in the belief that it sharpens recall and opens the door to lucid dreaming, the state in which a dreamer realizes they are dreaming. In a preliminary study, lucid dreamers managed to communicate with experimenters through eye movements or facial muscle signals, comprehending complex questions and using working memory.

  Sigmund Freud held that dreams are the symbolic expression of frustrated desires pushed into the unconscious mind, and he used dream interpretation through psychoanalysis to try to surface them. The body behaves oddly in REM as well. The parasympathetic nervous system grows more active, which can cause erection of the penis or clitoris, and in males 80 to 95 percent of REM sleep normally comes with partial to full penile erection. Yet only about 12 percent of men's dreams contain sexual content, and erections during sleep are no more frequent during sexual dreams than during any other kind.

• 8. Sleep Across Cultures

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  A. Roger Ekirch, a historian, traces the loss of segmented sleep to the urban upper class of late 17th-century Europe, a change that spread over the next 200 years. By the 1920s, in his words, the idea of a first and second sleep had receded entirely from our social consciousness. He pins the shift on street lighting, domestic lighting, and a surge in coffee houses, which slowly turned night into a legitimate time for activity. Before the Industrial Revolution, biphasic sleep was the norm, and in pre-light cultures people might sleep soon after sunset, then wake several times through the night, sometimes for hours.

  Sleep has long been likened to death. In Greek mythology, Hypnos, the god of sleep, and Thanatos, the god of death, were both children of Nyx, the goddess of night, and poets including John Donne, Samuel Taylor Coleridge, Percy Bysshe Shelley, John Keats wrote on the bond between the two. Long-sleep legends run deep, from Epimenides of Knossos, who the biographer Diogenes Laertius said slept fifty-seven years in a cave under Mount Ida, to the Seven Sleepers of Ephesus, who woke 360 years later. Washington Irving carried the theme into American letters with Rip Van Winkle, first published in 1819 in The Sketch Book of Geoffrey Crayon, Gent., whose hero sleeps twenty years through the American Revolution.