Catathrenia is a rare condition characterized by monotonous irregular groans occurring during sleep. Onset is usually in adolescence or early adulthood, and affected individuals are often unaware of their sleep-related groaning, which usually is noted by their bed partner or a family member. It should be differentiated from moaning during epileptic seizures, central sleep apnea, sleep-related laryngospasm, snoring and stridor. On a conventional PSG, it can be easily mistaken for central sleep apnea. The groaning sound is always preceded by inspiration which helps differentiate it from central apnea. A 20-year-old man was refered for making strange sounds during sleep since childhood. The sound appeared about 2 hours before wake-up time, lasted about 1 or 2 hours and disappeared after waking up. On PSG with video-audio recording, 5 sets of nocturnal groaning (from 3 to 12 times each) were noted from 1 o’clock to 4 o’clock (AM) and 1 sound persisted from 5 to 30 seconds. Apnea was noted and the AHI was 10/h (composed of central apnea index 8.7/h, obstructive apnea index 0.3/h, and hypopnea index 0.6/h). The mean and the lowest oxygen saturation were 97% and 94% respectively.