This chapter presents the clinical history, examination, follow-up, treatment, diagnosis, and the results of the procedures performed on a 24-year-old man who was admitted with the chief concern of abnormal breathing sounds during sleep for the past 5 years. Physical examination and vital signs were normal with a BMI of 28 kg/m2. His Epworth Sleepiness Scale score, for subjective assessment of excessive daytime somnolence, was 9. The patient entered sleep through NREM sleep stages, had recognizable NREM/ REM sleep alternations and physiological muscle atonia during REM sleep, with a total sleep time of 222 minutes. The groaning sounds lasted between 5 and 15 seconds and recurred in clusters, 16 minutes in net duration but spanning across 30 minutes. The overall clinical and polygraphic features in this patient were felt to be consistent with the diagnosis of catathrenia, a syndrome whose etiology remains unclear.