Sleep apnea (or sleep apnoea in British English; ) is a sleep disorder characterized by pauses in breathing or instances of shallow or infrequent breathing during sleep. Each pause in breathing, called an apnea, can last for several seconds to several minutes, and may occur 5 to 30 times or more in an hour. Similarly, each abnormally shallow breathing event is called a hypopnea. Sleep apnea is classified as a dyssomnia, meaning abnormal behavior or psychological events occur during sleep. When breathing is paused, carbon dioxide builds up in the bloodstream. Chemoreceptors in the blood stream note the high carbon dioxide levels. The brain is signaled to wake the person sleeping and breathe in air. Breathing normally will restore oxygen levels and the person will fall asleep again. Sleep apnea is often diagnosed with an overnight sleep test called a polysomnogram, or “sleep study”. There are three forms of sleep apnea: central (CSA), obstructive (OSA), and complex or mixed sleep apnea (i.e., a combination of central and obstructive) constituting 0.4%, 84%, and 15% of cases, respectively. In CSA, breathing is interrupted by a lack of respiratory effort; in OSA, breathing is interrupted by a physical block to airflow despite respiratory effort, and snoring is common. According to the National Institutes of Health, 12 million Americans have OSA. There are more cases of sleep apnea still because people either do not report the condition or do not know they have sleep apnea. Regardless of type, an 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 (sequelae). Symptoms may be present for years (or 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. Sleep apnea affects not only adults but some children as well. People have issues with excessive daytime sleepiness (EDS) and impaired alertness. In other words, common effects of sleep apnea include daytime fatigue, a slower reaction time, and vision problems. OSA may increase risk for driving accidents and work-related accidents. If OSA is not treated, one has an increased risk of other health problems such as diabetes. Even death could occur from untreated OSA due to lack of oxygen to the body. Moreover, patients are examined using “standard test batteries” in order to further identify parts of the brain that may be adversely affected by sleep apnea, including those that govern: “executive functioning”, the way the patient plans and initiates tasks paying attention, working effectively and processing information when in a waking state using memory and learning. Due to the disruption in daytime cognitive state, behavioral effects are also present. These include moodiness, belligerence, as well as a decrease in attentiveness and drive. Another symptom of sleep apnea is waking up in sleep paralysis. In severe cases, the fear of sleep due to sleep paralysis can lead to insomnia. These effects become very hard to deal with, thus the development of depression may transpire. There is also evidence that the risk of diabetes among those with moderate or severe sleep apnea is higher. There is also increasing evidence that sleep apnea may also lead to liver function impairment, particularly fatty liver diseases (see steatosis). Finally, because there are many factors that could lead to some of the effects previously listed, some patients are not aware that they suffer from sleep apnea and are either misdiagnosed, or just ignore the symptoms altogether.