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
Sleep spindles and K complexes occur in which phase of the sleep cycle 
Select and explain. 
A. REM, B. Stage I, C. Stage II, D. Stage III
C: Stage II
Answer: C: Stage II 
Explanation: Sleep spindles are sudden bursts of oscillatory brain activity generated during stage 2 of NREM sleep.K complexes are large waves that react to external stimuli while sleeping also seen during stage 2 of NREM sleep.