Academic Reading — All-Types Test 23

Reading passage

The Biology of Sleep A. Every animal that has ever been studied closely appears to sleep, or to do something that strongly resembles it. Even creatures without a recognisable brain, such as the jellyfish, pass through nightly spells of reduced activity from which they are slow to rouse. This near-universal pattern has long puzzled biologists, because sleep looks, on the surface, like a reckless waste of time. An organism that lies still for hours can neither feed nor reproduce, and it becomes an easy target for anything that hunts it. Whatever sleep does for the body, the benefit must be substantial enough to outweigh these considerable costs. B. The most familiar feature of human sleep is that it unfolds in stages rather than as a single uniform state. Over the course of a night, a sleeper cycles repeatedly between periods of deep, slow-wave rest and lighter phases marked by darting movements of the eyes beneath closed lids. It is chiefly during these latter phases that the most vivid dreaming occurs. Each full cycle lasts roughly ninety minutes, and the proportions shift as the night wears on: deep rest dominates the early hours, while the eye-movement phases lengthen towards morning. A person woken during a deep phase feels far groggier than one roused later, a difference that any parent of a young child soon learns to recognise. C. Why the body should insist on this elaborate architecture is a question that has produced several competing answers rather than one settled explanation. One influential proposal holds that sleep exists mainly to conserve energy, lowering the body's demands during the hours when foraging would be least profitable. Yet the savings turn out to be modest, no greater than those achieved by simply resting while awake, which makes energy conservation an unconvincing sole motive. A second and now widely favoured idea concerns the brain rather than the body. During waking hours the connections between nerve cells are strengthened wholesale as we absorb experience; sleep, on this view, prunes the weaker of these links, preserving what matters and discarding the rest so that the following day begins with a clean slate. D. Support for the restorative account has come from studies of the fluid that bathes the brain. Researchers at the fictional Halbrook Institute reported that the channels carrying this fluid widen noticeably during deep sleep, allowing a more vigorous flushing of the waste products that accumulate while a person is awake. Among the substances cleared away are proteins that, if left to build up, are associated with several disorders of memory in later life. The finding lent unexpected weight to the folk intuition that a troubled mind is helped by a good night's rest, and it suggested that the cleaning cannot be done properly while the brain is busy with the demands of the day. E. Whatever its deepest purpose, the timing of sleep is governed by an internal clock that runs on a cycle of close to, but not exactly, twenty-four hours. Left in permanent dim light, with no cues from sunrise or clocks, most people drift gradually out of step with the outside world, going to bed a little later each day. In ordinary life the clock is reset each morning by exposure to bright light, which is why travellers crossing several time zones feel so disoriented until their internal rhythm catches up. The hormone that signals the approach of night rises in the evening and falls before dawn, and artificial light late at night can blunt this signal, which is one reason that screens before bed are so often discouraged. F. The consequences of going without sleep are severe and, in extreme cases, fatal. Laboratory animals kept awake indefinitely sicken and die within weeks, though the precise mechanism of death remains disputed. In humans, even a single poor night dulls attention, slows reaction times and sours the mood, while chronic shortage has been linked to a broad range of ailments from weakened immunity to disordered metabolism. Such findings have gradually shifted the popular view of sleep from an indulgence, to be trimmed whenever life grows busy, into something closer to a biological necessity on a par with food and water.