Sleep sex encompasses various human sexual behaviors occurring in or from the sleep state, or appearing to do so. Although uncommon, it is mostly an unusual variant of the non-rapid eye movement parasomnias, sleepwalking and confusional arousal. In clinical circumstances, as with other more common parasomnias, the diagnosis is heavily informed by a corroborated history, aided where appropriate by physiologic, imaging, and other investigations, and often involving formal psychiatric and psychological assessment. The treatment of sleep sex needs to be individualized to the circumstances of each case. In the forensic context, sleep sex creates challenges for the medical expert witness.