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It is characterized by pauses in breathing during sleep.

These episodes, called apneas (literally, "without breath"), each last long enough that one or more breaths are missed, and occur repeatedly throughout sleep.

In obstructive sleep apnea, breathing is interrupted by a physical block to airflow, despite the effort to breathe. The individual with sleep apnea is rarely aware of having difficulty breathing, even upon awakening.

Sleep apnea is recognized as a problem by others witnessing the individual during episodes or is suspected because of its effects on the body.

Symptoms may be present for years, even decades without identification, during which time the sufferer may become conditioned to the daytime sleepiness and fatigue associated with significant levels of sleep disturbance. Since the muscle tone of the body ordinarily relaxes during sleep, and since, at the level of the throat, the human airway is composed of walls of soft tissue, which can collapse, it is easy to understand how breathing can be obstructed during sleep.

Although many individuals experience episodes of obstructive sleep apnea at some point in life, a much smaller percentage of people are afflicted with chronic, severe obstructive sleep apnea. Many people experience elements of obstructive sleep apnea for only a short period of time.

This can be the result of an upper respiratory infection that causes nasal congestion, along with swelling of the throat, or tonsillitis that temporarily produces very enlarged tonsils.

The Epstein-Barr virus, for example, is known to be able to dramatically increase the size of lymphoid tissue during acute infection, and obstructive sleep apnea is fairly common in acute cases of severe infectious mononucleosis.

Temporary spells of obstructive sleep apnea syndrome may also occur in individuals who are under the influence of a drug (such as alcohol) that may relax their body tone excessively and interfere with normal arousal from sleep mechanisms.