The monitoring of oxygenation and ventilation is important in the assessment of sleep-related breathing disorders. However, because oxygen values can change often during the course of sleep, repeated measurements must be taken to ensure accuracy. The direct measurements of arterial oxygen tension only offer a static glimpse, and repeated measurements from invasive procedures such as sampling arterial blood for oxygen will disturb the patient's sleep; therefore, noninvasive methods are preferred such as pulse oximetry, transcutaneous oxygen monitoring, transcutaneous carbon dioxide, and pulse transit time.