Three goups, each composed of five individuals were investigated: students, patients and controls aged 20--45 years. The characteristic features of the first sleep in the laboratory were compared with the sleep during experimental pre-examination stress in students and preoperative stress in patients. It was found that the first sleep in the laboratory had the same features as the sleep in experimental stress. The process of adaptation to the conditions of sleep in the laboratory during the 2nd and 3rd night was studied. In the case of patients no adaptation was observed during three successive nights.