Polysomnography is a non-invasive and painless test that allows the study of sleep stages (REM and non-REM) and its various fluctuations. It is performed to diagnose disorders such as obstructive sleep apnoea, narcolepsy, excessive sleepiness, restless legs syndrome or sleep terrors, among others. Generally, polysomnography is done at night, in specialised centres or at the patient’s home. While the patient sleeps electrodes on the chin, scalp. the outer edge of the eyelids, and on the chest record variables such as blood pressure, EEG activity, and eye, muscle, heart and lung activity. It measures the time it takes the patient to fall asleep and duration of different sleep stages. A specialist observes the sleeping patient from another room, and a polygraph records and stores the information of the different physiological signals, which will then be analysed for a diagnosis. The sleep period might also be recorded on video for later analysis.