A decrease in blood oxygen levels below normal during sleep is called sleep-related hypoxemia. Blood oxygen levels can be measured invasively by an arterial blood gas test or noninvasively by wearing a pulse oximeter. Oxygen levels below 90 are considered hypoxemia. Symptoms may include increased heart rate, shortness of breath, fatigue, and sleepiness during the daytime. The severity of the hypoxemia affects the severity of the symptoms. In some cases, untreated sleep-related hypoxemia can be a cause of death.
