Summary Non‐24‐h sleep–wake rhythm disorder is quite rare in sighted patients and frequently associated with psychiatric disorders. We report the case of a 46‐year‐old man with autism spectrum disorder (ASD) and agoraphobia who had been referred for a suspicion of obstructive sleep apnea syndrome (OSAS). Polysomnography and arterial blood gas confirmed moderate OSAS associated with hypoventilation. Continuous positive airway pressure (CPAP) was started on fixed mode with excellent results. At follow‐up, his CPAP report data revealed an irregular sleep–wake rhythm with a progressive offset of sleep schedule and wake time delayed from 1 h from day to day. Melatonin (or agonist) is efficacious and safe for long‐term treatment in ASD and circadian rhythm sleep–wake disorder (CRSWD) with light therapy and wakefulness promoting medication. This case underlines the importance to sensitise psychiatrists to sleep and CRSWD, and also that CPAP data offer a possible objective alternative to sleep diary.