Academic Reading — All-Types Test 8

Reading passage

The Biology of Sleep Sleep is one of the most familiar features of animal life, yet for a long time it remained among the least understood. Every night, humans surrender roughly a third of their existence to a state in which awareness of the surrounding world all but disappears. Far from being a passive shutting-down of the body, sleep is now recognised as an active and highly organised process, governed by structures deep within the brain and tuned by a clock that runs on a cycle of about twenty-four hours. The study of why animals sleep, and what happens while they do, has become one of the more productive areas of modern biology. The most obvious driver of sleep is the internal timekeeper known as the circadian rhythm. In mammals this rhythm is set by a small cluster of cells in a region called the suprachiasmatic nucleus, situated just above the point where the optic nerves cross. These cells receive information about light directly from the eyes and adjust the body's schedule accordingly. When light fades in the evening, a gland deep in the brain begins to release a hormone called melatonin, which nudges the body towards rest. The rhythm is remarkably persistent: volunteers kept in caves or windowless rooms, with no clues about the time of day, continue to sleep and wake on a roughly regular cycle, although it tends to drift slightly longer than a natural day. A second force works alongside the clock and is often described as sleep pressure. The longer an animal stays awake, the stronger its need for sleep becomes. This mounting pressure is linked to the gradual build-up of certain chemicals in the brain during waking hours, one of the best known being a compound called adenosine. As adenosine accumulates, drowsiness deepens; during sleep the compound is cleared away, and the pressure eases. Caffeine, the world's most widely consumed stimulant, works by blocking the receptors that adenosine would otherwise occupy, which is why a cup of coffee can hold tiredness at bay for a few hours without actually removing the underlying need for rest. Sleep itself is not a single uniform condition but a sequence of distinct stages that repeat through the night. Broadly, these fall into two categories. The first, known as non-REM sleep, includes the deepest and most restorative phases, during which brain activity slows into large, rolling waves and the body's repair processes are especially active. The second, called REM sleep after the rapid eye movements that accompany it, is the phase most closely associated with vivid dreaming. During REM sleep the brain is almost as active as it is in waking, yet the muscles of the body are temporarily paralysed, a safeguard that prevents a sleeper from acting out the events of a dream. A full cycle through these stages lasts roughly ninety minutes, and a typical night contains four or five such cycles. The purposes served by these stages appear to be numerous. One well-supported function concerns memory. During deep non-REM sleep the brain is thought to replay and consolidate the experiences of the day, transferring fresh memories from temporary storage into more durable form, while REM sleep may help weave new information together with older knowledge. Another function is physical maintenance: growth hormone is released in greater quantities during deep sleep, tissues are repaired, and the immune system is reinforced. More recently, researchers have described a kind of nightly cleaning process, in which spaces between brain cells widen during sleep and fluid flushes away waste products that build up during the day. The accumulation of such waste has been linked, in some studies, to disorders of the ageing brain. Given these roles, it is unsurprising that going without sleep carries real costs. Even a single night of poor rest can blunt concentration, slow reaction times and sour the mood, while longer periods of deprivation impair judgement and weaken the body's defences against illness. The amount of sleep an individual needs changes across a lifetime: newborn infants may sleep for sixteen hours or more, much of it in the REM phase, whereas most adults settle at somewhere between seven and nine hours. What remains constant is the underlying message of the research. Sleep is not an optional luxury to be trimmed away when life grows busy, but a biological necessity as fundamental to health as food and water, quietly shaping the mind and body night after night.