Volunteers who are eligible following screening will be invited to return to the sleep clinic so that they may undergo a one-night period of acclimatisation to the study procedures (the polysomnography [PSG] screen, Visit 2). Following confirmation that the participant has adhered to the protocols restrictions, including tests for drugs of abuse and alcohol use, vital signs will be measured and the actigraph will be replaced. Twice before going to sleep and once on awakening, participants will complete a Karolinska Sleepiness Scale and undergo a Karolinska Drowsiness Test. Participants will then undergo polysomnography, a series of recordings of brain activity using electroencephalography and patterns of breathing, for an eight hour period of sleep. In the evening a cannula will be fitted so that repeat blood samples can be taken and this will be removed the next morning. During the course of the evening three urine samples and four blood samples will be taken for the purpose of assay for melatonin. Three additional blood samples will be taken during the night and urine will be collected. On awakening, urine will be collected and the participant's stability will be assessed using the Romberg test and the Heel-to-Toe Gait test.