List four stages of sleep and a key characteristic of each.

• A. Stage N1—light sleep with theta waves; N2—sleep spindles and K-complexes; N3—slow-wave delta sleep; REM—rapid eye movements and dreaming.
• B. Stage 1—deep sleep with delta activity; Stage 2—alpha waves; Stage 3—REM; Stage 4—wakeful.
• C. Stage 1—alert; Stage 2—REM; Stage 3—dreaming; Stage 4—light sleep.
• D. Stage N1—beta waves; N2—theta waves; N3—gamma bursts; REM—no dreaming.

Answer: (A)

Understanding sleep architecture means recognizing four stages of sleep and the distinctive brain activity and features that accompany each. The description that best fits is: Stage N1 is light sleep with theta waves, reflecting the transition from wakefulness into sleep. Stage N2 is a deeper stage characterized by sleep spindles and K-complexes, which help guard against waking and mark a continuation into sleep. Stage N3 is the deepest non-REM stage and is dominated by slow, high-amplitude delta waves, representing restorative, quiet brain activity. REM sleep features rapid eye movements and, despite the name, a brain that is quite active and often vivid dreaming.

These patterns align with how sleep cycles unfold across the night, typically moving from lighter to deeper stages and then into REM in roughly 90-minute cycles. The other descriptions mix up the markers—assigning deep sleep to wakeful or lightly awake stages, or labeling alpha, beta, or gamma activity where theta, spindle/K-complex activity, and delta waves are the characteristic signs of the respective stages—so they don’t match how sleep stages are defined.