Normally a healthy individual sleeps for 4 to 10 hours with 4 - 5 alternating and recurrent 90 to 110 minute cycles of deep and lighter sleep. The deep stage of sleep is most important and corresponds to healthy sleeping of the person. But in case of OSA, the person spends an excessive amount of time in the lighter stages of sleep leading to disturbed or lack of sleep in the night. They may also experience repeated nighttime awakenings. All these factors eventually lead to excessive daytime sleepiness and lethargy. 25% of your total sleep should be REM sleep. But patients with OSA would not achieve even 10% REM. Their sleep quantity maybe constant but quality is poor. It is carried over to the next day.