This overnight test is used to help diagnose obstructive sleep apnea. The patient takes a portable apnea recorder home from the Surrey Sleep Laboratory. These devices usually record nasal airflow, respiratory effort, pulse oximetry and body position. The test may underestimate or overestimate the degree of sleep apnea. The devices do not record EEG and have no mechanism to determine the sleep quantity or quality of sleep which leads to incorrect measure of the sleep apnea. The devices do not help diagnose other sleep disorders and may wrongly attribute the day time sleepiness to sleep apnea.