IELTS Reading: Table Completion

Human sleep is not a uniform state of unconsciousness but a dynamic process composed of several distinct stages that cycle throughout the night. Sleep researchers divide these stages into two broad categories: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep itself comprises three stages. Stage 1, also called N1, is a light transitional stage lasting only one to seven minutes in a full sleep cycle. During N1, theta brainwaves predominate, muscle activity decreases, and the sleeper can be easily aroused. This stage accounts for roughly 5% of total sleep time. Stage 2, or N2, is characterised by the appearance of sleep spindles — bursts of high-frequency neural activity — and K-complexes, which are thought to suppress arousal and consolidate memory. N2 lasts approximately 25 minutes in the first cycle and constitutes about 45–55% of total nocturnal sleep. Importantly, N2 is when the body's core temperature begins to drop. Stage 3, formerly divided into Stages 3 and 4 but now consolidated, is deep slow-wave sleep (SWS) or delta sleep. Brain activity slows dramatically, producing high-amplitude delta waves. Arousal during SWS is difficult, and a person woken from this stage typically experiences sleep inertia — a feeling of grogginess that may last up to 30 minutes. SWS is most restorative physically: growth hormone secretion peaks during this stage, and immune function is supported. SWS accounts for 15–25% of sleep and is most abundant in the first half of the night. REM sleep, the fourth stage, is fundamentally different from NREM. The brain is highly active, producing patterns similar to wakefulness, yet the body's voluntary muscles are actively paralysed — a mechanism thought to prevent acting out dreams. Dreams are most vivid and narrative during REM. Each REM episode lasts about 90–120 minutes and becomes progressively longer through the night. REM constitutes 20–25% of adult sleep and is associated with emotional regulation, procedural memory consolidation, and creative thinking.