Many patients with a traumatic brain injury (TBI) experience a range of sleep problems. Although some studies investigated the pathophysiology of sleep-wake cycle disturbances in TBI patients, it has not been clarified. This paper presents a middle aged female patient who showed sleep deprivation and sleep-wake cycle disturbances for approximately three months after TBI. The improvement in the subjective and objective sleep quality was shown by the sleep diary and actigraphy during this period. Moreover, the dim light melatonin onset (DLMO) had been delayed before returning to the normal range in 3 months. In addition, the patient showed an improvement in the neurocognitive function, including attention, memory and language function, along with a consolidation of the sleep-wake cycle. This case showed that the sleep disturbance following a TBI was probably caused by the disrupted melatonin rhythm based on the abnormality of the DLMO. In addition, the cognitive dysfunction after TBI could be associated with sleep-wake cycle disturbances because its gradual improvement occurred as the sleep disturbance diminished. Further studies on the change in circadian rhythm after a brain injury related to neurocognitive impairment are required.