Nocturnal intrinsic disorders of sleep are much more common than hitherto assumed. In middle-aged men, a prevalence of 0.3 to 3% of severe obstructive sleep apnea syndromes necessitating treatment is expected. The international classification of sleep disorders (ICSD 1990) contains the definitions and descriptions of the different entities. A patient with combined obstructive sleep apnea and a hypoventilation syndrome due to obesity serves to illustrate problems with diagnosis and particularly treatment of this respiratory disorder during sleep. The most frequent sleep disorders are briefly presented and placed in the context of other concomitant somatic diseases. Because of the possible serious course of nocturnal hypoxemias, the conclusion is justified that a timely and accurate diagnosis is essential for the patient. Transcutaneous oximetry during the night is sufficient as screening procedure. It can be applied to outpatients. In depth, evaluation should be carried out at a specialized center with polysomnography.