The study is a collaborative project between sleep specialists at Papworth Hospital and ophthalmologists at Hinchingbrooke Hospital and will involve patients attending glaucoma clinic and their relatives who will act as a control group. Two study visits will be required on consecutive days. Participants will undergo a comprehensive ocular examination and a sleep study in the form of respiratory polysomnography (respiratory PSG) at home. A brief medical history focused on cardiovascular and metabolic co-morbidities will be taken.