**2. Sleep stages**

During normal sleep, the stages are:

**1st stage** is non-rapid eye movement (non-REM), lasting 70-90 minutes. The non-REM is the quiet sleep, which is consists of 4 stages.45-50% of total sleep is stage 2. Delta sleep is stages 3 and 4, which are a deeper sleep marked by the increasing appearance of high-amplitude slow waves. There is generalized slowness in all activities in non-REM sleep stage.

**2nd stage** rapid eye movement (REM) stage will follow for 20 minutes. REM stage represents the deep sleep stage; it follows the non-REM stage with 20-25% of total sleep. During average night REM to non-REM ratio is 4:6 with intervals 60-90 minutes. Physiologic changes during REM are generalized muscle atonia except for ocular muscles, increase temperature, blood flow and oxygen use in the brain as well as increase in heart rate, blood pressure and respiration with dramatic fluctuations. Respiration is controlled by 2-control systems; metabolic and behavioral. Non-REM is predominantly controlled by the metabolic control system, which is influenced by hypoxia and hypercapnia. On the other hand during the REM sleep, behavioral control system is predominant. OSA usually occurs during stage 3, 4 and REM, which are the deep sleep stages, and that is because of blunt responses to hypoxia and hypercapnia along with the generalized muscle atonia; pharyngeal wall muscles may collapse [1] [2].
