Which of the following is a feature of stage 2 sleep?

• A. Sleep spindles
• B. Theta waves
• C. K complexes
• D. Hypnic jerk

Answer: (C)

Stage 2 sleep is defined by distinctive EEG features, especially K complexes and sleep spindles. Among these, K complexes are a classic hallmark of this stage—large, sudden waveforms that often occur in response to stimuli and help protect sleep by limiting arousal. Theta waves are characteristic of lighter sleep and are more prominent in earlier stages, not the defining signal of stage 2. Sleep spindles also mark stage 2, but the presence of a K complex is the most distinctive feature used to identify this stage. Hypnic jerks happen at sleep onset, not during stage 2.