Type I polysomnography, a sleep study performed overnight while being continuously monitored by a credentialed technologist, is a comprehensive recording of the biophysiological changes that occur during sleep. It is usually performed at night, when most people sleep, though some labs can accommodate shift workers and people with circadian rhythm sleep disorders and do the test at other times of the day. The PSG monitors many body functions, including brain activity (EEG), eye movements (EOG), muscle activity or skeletal muscle activation (EMG), and heart rhythm (ECG), during sleep. After the identification of the sleep disorder sleep apnea in the 1970s, the breathing functions, respiratory airflow, and respiratory effort indicators were added along with peripheral pulse oximetry. Polysomnography no longer includes NPT, Nocturnal Penile Tumescence, for monitoring of erectile dysfunction, as it is reported that all male patients will experience erections during phasic REM sleep, regardless of dream content. Limited channel polysomnography, or unattended home sleep tests, should be referenced as Type II – IV channel polysomnography. With some controversy, polysomnography is best performed by technicians and technologists who are specifically licensed and accredited in sleep medicine. However, at times nurses and respiratory therapists are allowed to perform polysomnography despite lack of specific knowledge and training in this area.