BACKGROUND: Sleep disturbances are common after acquired brain injury. Sedatives can exacerbate behavioural disorders. OBJECTIVES: This study reports the case of a severely brain damaged man (TM) who developed a non-24 hour sleep cycle disorder that was effectively managed by the administration of a melatonin receptor agonist, agomelatine. METHOD: TM suffered significant brain damage as a result of a large subarachnoid haemorrhage of his right anterior cerebral artery complicated by midline shift and subsequent infarction of his left middle cerebral artery. In addition to challenging behaviour and cognitive impairment, TM presented with a recurrent disturbed sleep-wake pattern that significantly worsened his quality-of-life. He was diagnosed as suffering of non-24 hour sleep-wake disorder. Challenge was recorded using the Overt Aggression Scale Modified for Neuro-Rehabilitation (OASMNR). RESULTS: Typical hypnotics had no or ill effects. Agomelatine prescription (25 mg) led to significant OASMNR and sleep efficiency change with effects apparent at 1.5 years later. CONCLUSIONS: Administration of the melatonin receptor (MT₁ and MT₂) agonist agomelatine each night resulted in an immediate and sustained improvement on sleep and on indices of challenging behaviour.