This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
In which phase of sleep, does the bruxism occur? 
Select and explain. 
A. NREM 2, B. NREM 3, C. REM sleep, D. NREM 4
A: NREM 2
Answer: A: NREM 2 
Explanation: Bruxism occurs in both stage I and stage II. But more common in stage II.
Note: OSA is not associated with a reduction in total sleep time, but individuals with OSA experience a much greater time in stage 1 NREM sleep (from an average of 10% of total sleep to 30–50%) and a marked reduction in slow-wave sleep (stages 3 and 4 NREM sleep). The pathophysiology of OSA includes both a reduction in neuromuscular tone at the onset of sleep and a change in the central respiratory drive.
Reference: Ganong’s Review of Medical Physiology T W E N T Y - F I F T H   E D I T I O N page no 276