In stage 2 sleep, which phenomenon consists of short, rapid bursts of high-frequency activity?

• A. Alpha waves
• B. Delta waves
• C. Sleep spindles
• D. K complexes

Answer: (C)

Stage 2 sleep is marked by sleep spindles, which are brief bursts of fast, rhythmic brain activity. They occur at about 11–16 Hz and last roughly 0.5 to 2 seconds. These thalamocortical bursts help gate sensory input, protecting sleep and supporting early memory processing. The other patterns don’t fit the description: alpha waves are slower activity seen with relaxed wakefulness; delta waves are slow waves that dominate deep sleep; K complexes are large, single deflections rather than rapid bursts. So the phenomenon described is sleep spindles.