Verb Tense Practice Test

PASSAGE III: The Architecture of Sleep

¹ Sleep is often viewed as a passive state, a time when the body shuts down to conserve energy. In reality, however, sleep is a highly active and complex biological process. While we rest, our brains are engaging in a neurological workout that is essential for physical health and mental clarity. This nightly maintenance is (23) governed by two opposing forces: the sleep drive, which builds up the longer we are awake, and the circadian rhythm, an internal "clock" that regulates our sleep-wake cycle based on light and darkness.

² Once asleep, the brain cycles through four distinct stages. The first three stages are known collectively as non-rapid eye movement (NREM) sleep. During the deepest part of NREM, (24) heart rate slows down, blood pressure drops, and the body repairs tissues. (25) Conversely, the fourth stage is rapid eye movement (REM) sleep. In this phase, brain activity spikes to levels similar to wakefulness. This is the stage where dreaming occurs and (26) memories were consolidated from short-term to long-term storage.

³ One of the most critical functions of sleep is "taking out the trash." For years, scientists wondered how the brain, (27) unlike other organs, cleared out metabolic waste products. In 2013, researchers discovered the "glymphatic system," a macroscopic waste clearance system formed by glial cells. During deep sleep, these cells shrink, increasing the space between neurons by up to 60 percent. This allows cerebrospinal fluid (28) to flow rapid through the brain tissue, flushing out toxic proteins associated with neurodegenerative diseases. (32)

⁴ (29) Neglecting this biological need has serious consequences. Sleep deprivation impairs cognitive function, weakens the immune system, and increases the risk of chronic conditions like heart disease. (30) Experts recommend adults get between seven and nine hours of sleep per night. Despite these risks, modern society often treats sleep as a luxury rather than a necessity. Artificial lighting and digital screens (31) disrupts the production of melatonin, the hormone that signals to the body that it is time to rest.

⁵ We cannot cheat our biology. Caffeine and sheer willpower can only mask the symptoms of fatigue for so long. Eventually, the "sleep debt" must be repaid. By prioritizing sleep, we are not being lazy; we are investing in our long-term health. The architecture of sleep is built on a foundation of millions of years of evolution, and (33) it remains the most effective medicine we have. (34)