Sleep apnea, also spelled sleep apnoea, is a sleep disorder characterized by pauses in breathing or instances of shallow breathing during sleep. Each pause in breathing, called apnea, can last for several seconds to several minutes. By definition, more than five episodes an hour must occur. Similarly, each abnormally shallow breathing event is called a hypopnea. Sleep apnea is classified as a dyssomnia. 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".