In which phase of sleep, does the bruxism occur?

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