Question

Passage A
For decades, evolutionary biologists and neuroscientists have sought to unravel the enigma of sleep—a state of profound physical vulnerability and reduced environmental responsiveness. Because this dormant state severely compromises an organism's ability to forage, mate, or evade predators, it must offer significant, offsetting compensatory advantages to have persisted across virtually all animal species. The most compelling explanation posits that sleep serves a critical, localized restorative function for the central nervous system. During waking hours, normal metabolic processes naturally produce toxic biochemical byproducts, such as beta-amyloid, which progressively accumulate in the interstitial spaces of the brain. The brain, which consumes a disproportionate amount of the body's energy, is uniquely susceptible to this toxic buildup. Recent studies involving advanced imaging of the glymphatic system reveal that during slow-wave sleep, the brain's glial cells contract significantly, dramatically increasing the flow of cerebrospinal fluid. This fluid acts as a biological rinse, actively washing away accumulated neurotoxins. From this perspective, the primary evolutionary driver for sleep is foundational somatic and neural maintenance. Without this nightly clearance, cellular function degrades rapidly, inevitably leading to acute cognitive impairment and, eventually, severe long-term neurodegeneration. Thus, despite the profound inherent risks of entering a disconnected state, sleep is an unavoidable biological mandate dictated entirely by the severe metabolic costs of maintaining a complex, active nervous system.

Passage B
While traditional theories of sleep have frequently focused on straightforward physical recuperation and energy conservation, contemporary neurocognitive research strongly suggests that sleep’s primary evolutionary function is far more active, dynamic, and complex than mere physiological restoration. Rather than acting strictly as a period of biological downtime or passive recovery, sleep represents a crucial, highly orchestrated phase of neural plasticity and sophisticated information processing. Throughout the waking hours, an animal's brain continuously acquires vast amounts of new environmental information, forming fragile, temporary synaptic connections. However, it is primarily during the specific architectural stages of sleep—particularly Rapid Eye Movement (REM) and deep non-REM sleep—that these transient daytime memories are selectively reactivated, deeply consolidated, and seamlessly integrated into long-term cortical networks. This intricate neurological process not only stabilizes essential memories but also actively prunes extraneous or overlapping neural connections, thereby optimizing the brain's finite capacity for future learning and environmental adaptation. Furthermore, sleep facilitates abstract problem-solving by allowing the brain to reorganize acquired information entirely free from the constant, demanding interference of waking sensory input. Therefore, despite the apparent disadvantages of extended periods of unconsciousness, the widespread persistence of sleep across the animal kingdom is best understood not simply as a blunt mechanism for recovering from the day's physical toll, but as a highly sophisticated, essential cognitive adaptation fundamentally required for memory consolidation and adaptive behavioral modification.

Which one of the following most accurately describes the central purpose of both passages?