There are two main categories of sleep:
NREM consists of three phases, including the transitional phase between wakefulness and sleep that lasts for one to seven minutes and accounts for around 5% of total sleep time; the light sleep phase, which makes up about half of total sleep time; and the slow wave phase, or Delta phase, which is a deeper and more relaxed sleep. It makes up about 20% of total sleep time.
REM is a period of deep, refreshing sleep. During this time, eyes move side to side and the skeletal muscles are nearly paralized. This represents 25% of the total sleep time and is the stage where dreaming occurs. REM paralysis can result in a loss of muscle tone in the upper airway.
People who suffer from obstructive sleep apnea have a fragmented sleep architecture, or sleep cycle, which means they spend too much time in the lighter stages of sleep vs. the Delta Phase and REM stages. When it occurs repeatedly and sleep is not refreshing, excessive daytime sleepiness can result.
Apnea is defined as a period when breathing stops for 10 seconds or more. If the patient has more than five episodes of apnea per hour of sleep, it’s considered obstructive sleep apnea. When the decrease in airflow leads to a 4% decrease in oxyhemoglobin concentration in the blood, it’s referred to as Hypopnea.