Polysomnography (PSG) is a standard procedure for investigating sleep functions including behavior. This technique is described. The diagnostic value of PSG is shown by statistical data from the literature and our own patient sample which includes 442 insomniacs. In one out of five patients, insomnia is related to a known somatic factor, i.e. periodic movements in sleep or sleep apnea, which can only be reliably identified by PSG. A more differentiated analysis shows that PSG yields important diagnostic information in 50 to 75% of insomniacs. Three cases are reported, showing that even patients without clinical indications for sleep disturbance can have deficient sleep functions. If they can be specifically addressed, this may be the turning point of the treatment. Therefore, a wide application of PSG is favored.