Obstructive sleep apnea (OSA) is the most common type of sleep apnea and is caused by complete or partial obstructions of the upper airway. It is characterized by repetitive episodes of shallow or paused breathing during sleep, despite the effort to breathe, and is usually associated with a reduction in blood oxygen saturation. These episodes of decreased breathing, called "apneas" (literally, "without breath"), typically last 20 to 40 seconds. Individuals with OSA are rarely aware of difficulty breathing, even upon awakening. It is often recognized as a problem by others who observe the individual during episodes or is suspected because of its effects on the body. OSA is commonly accompanied with snoring. Some use the term obstructive sleep apnea syndrome to refer to OSA which is associated with symptoms during the daytime. Symptoms may be present for years or even decades without identification, during which time the individual may become conditioned to the daytime sleepiness and fatigue associated with significant levels of sleep disturbance. Individuals who generally sleep alone are often unaware of the condition, without a regular bed-partner to notice and make them aware of their symptoms. As the muscle tone of the body ordinarily relaxes during sleep, and the airway at the throat is composed of walls of soft tissue, which can collapse, it is not surprising that breathing can be obstructed during sleep. Although a very minor degree of OSA is considered to be within the bounds of normal sleep, and many individuals experience episodes of OSA at some point in life, a small percentage of people have chronic, severe OSA. Many people experience episodes of OSA for only a short period. 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 OSA is fairly common in acute cases of severe infectious mononucleosis. Temporary spells of OSA 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.