Which stage of NREM sleep is defined by regular bursts of waves that progressively increase and then decrease in amplitude?

• A. NREM stage 1
• B. NREM stage 3
• C. NREM stage 2
• D. Paradoxical sleep

Answer: (C)

Waxing and waning bursts of fast activity describe sleep spindles, which define NREM sleep stage 2. In this stage you move from lighter sleep toward deeper sleep, and the EEG shows these brief 12–14 Hz spindle bursts that rise and fall in amplitude. Stage 2 often also features K-complexes, but the key marker here is the spindle pattern. This distinguishes it from stage 1 (lighter sleep with simpler theta activity), stage 3 (dominant slow delta waves), and REM sleep (paradoxical sleep with mixed frequencies and rapid eye movements). The described bursts are a classic sign of stage 2 sleep.