Obstructive sleep apnea (OSA) is the most common sleep-related breathing disorder and is characterized by recurrent episodes of complete or partial obstruction of the upper airway leading to reduced or absent breathing during sleep. These episodes are termed "apneas" with complete or near-complete cessation of breathing, or "hypopneas" when the reduction in breathing is partial. In either case, a fall in blood oxygen saturation, a disruption in sleep, or both may result. A high frequency of apneas or hypopneas during sleep may interfere with restorative sleep, which – in combination with disturbances in blood oxygenation – is thought to contribute to negative consequences to health and quality of life. The terms obstructive sleep apnea syndrome (OSAS) or obstructive sleep apnea–hypopnea syndrome (OSAHS) may be used to refer to OSA when it is associated with symptoms during the daytime (e.g. excessive daytime sleepiness, decreased cognitive function).Most individuals with OSA are unaware of disturbances in breathing while sleeping, even after awakening. A bed partner or family member may observe an individual snoring or appear to stop breathing, gasp, or choke while sleeping. Individuals who live or sleep alone are often unaware of the condition. Symptoms may be present for years or even decades without identification, during which time the individual may become conditioned to the daytime sleepiness, headaches and fatigue associated with significant levels of sleep disturbance. Obstructive sleep apnea has been associated with neurocognitive morbidity, as well as a link between snoring and neurocognitive disorders.
The cycle of muscle-tone loss and restoration coinciding with periods of deep and light sleep repeats throughout the patients period of sleep. The number of apnoea and hypopnoea episodes during any given hour is counted and given a score. If a patient has an average of five or more episodes per hour, mild OSA may be confirmed. An average of 30 or more episodes per hour indicates severe OSA. Pathophysiological models
The causes of spontaneous upper-airway blockage are strongly debated by clinical professionals. The areas of thought are divided mostly into three medical groups. Some pulmonologists and neurologists believe the risk factors to be: 

Advanced age, although OSA occurs in neonates, as with Pierre Robin syndrome, and in all age groups of people. Brain injury (temporary or permanent), although this does not account for the 99% of OSA patients who have normal brains and normal lives. Decreased muscle tone caused by drugs or alcohol, or caused by neurological disorders. This also would not account for the majority of people with OSA. Long-term snoring, which is postulated to potentially induce local nerve lesions in the soft tissues of the pharynx. Snoring may produce traumatic vibrations that may give rise to nerve injuries in the upper airway muscles, further contributing to OSA.