IELTS Reading: Matching Headings

Section A For most of human history, sleep was considered a passive state — a simple absence of wakefulness in which the brain merely rested. This view began to change dramatically in the 1950s when American researchers Eugene Aserinsky and Nathaniel Kleitman discovered rapid eye movement (REM) sleep using electroencephalography. Their finding that the brain was as active during REM sleep as during wakefulness fundamentally overturned the prevailing model and launched the modern scientific study of sleep. Section B Sleep serves a complex array of biological functions that are still the subject of active research. The most well-supported theories hold that sleep is essential for memory consolidation — the process by which newly acquired information is transferred from short-term to long-term storage — and for synaptic homeostasis, the pruning and maintenance of neural connections formed during waking hours. Sleep deprivation studies consistently demonstrate profound impairments in attention, working memory, and complex decision-making, with deficits accumulating progressively over consecutive nights of insufficient sleep. Section C Beyond cognitive repair, sleep appears to play a critical role in metabolic regulation. During deep slow-wave sleep, the brain's waste-clearance system — a network of fluid channels known as the glymphatic system — becomes dramatically more active, flushing out protein waste products including beta-amyloid and tau, the proteins associated with Alzheimer's disease. Individuals who consistently sleep fewer than six hours per night have significantly elevated levels of these proteins, raising the possibility that chronic sleep deprivation is a modifiable risk factor for neurodegenerative disease. Section D Society's relationship with sleep has changed profoundly over the past century, largely driven by electrification and, more recently, the proliferation of digital screens. Artificial light — particularly the short-wavelength blue light emitted by LED screens — suppresses the secretion of melatonin, the hormone that signals the biological onset of night, shifting the internal clock later and delaying sleep onset. Studies of pre-industrial societies, in which individuals retire roughly two hours after sunset and wake before sunrise, suggest that the eight-hour consolidated sleep block typical of Western societies may be a relatively recent cultural construct. Section E Despite the universality of sleep across the animal kingdom, scientists remain uncertain about its evolutionary origins. Most animals sleep in some form, and even invertebrates such as fruit flies and nematode worms show rest states with many features analogous to sleep. One leading hypothesis holds that sleep evolved primarily as an energy-conserving adaptation, reducing metabolic demands during periods when activity would be inefficient or dangerous. An alternative view proposes that the brain maintenance and consolidation functions of sleep were the primary selective pressures — with energy conservation as a secondary benefit.