The authors propose a method of quantitative evaluation of excessive diurnal sleepiness intensity. They perform in each patient a 45-minute polygraphic examination to evaluate the occurrence of manifestations of wakefulness as well as all forms and stages of sleep, together with their latencies and total durations. In this way it is possible to describe the patients' sleepiness both quantitatively and qualitatively. The above test was used in the study of 8 healthy controls, 8 patients with narcolepsy-cataplexy, 8 patients with idiopathic hypersomnia and 8 patients suffering from the syndrome of hypersomnia with sleep apnea. All three groups of patients differed significantly from the control group showing deeper sleep stages of shorter latency and longer total duration. The three groups of patients differed also in some aspects from each other.