This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous System, Subject name: Physiology
During which of the following sleep stages shows sleep spindles in ECG? 
Select and explain. 
A. Stage 1, B. Stage 2, C. Stage 3, D. Stage 4
B: Stage 2
Answer: B: Stage 2 
Explanation: Ans. B. Stage 2Stage 1 (sleep)presence of theta waves 4-HzStage 2 (sleep)Sleep spindles appears as sinusoidal waves occurring at a frequency of 12-14 Hz. Presence of sleep spindles and K complexes. K complexes are occasional high voltage biphasic waves.stage 3A high-amplitude delta rhythm (0.5-4 Hz) are seen.Stage 4presence of slow waves.