Sudden nosebleed while sleeping is considered to be caused by dryness of nasal mucosa, hypertension and nasopharyngeal cancer. 1. Nasal mucous membrane dryness: the environment is too dry, which leads to rapid evaporation of nasal water, and there may be dryness, itching and other discomforts of the nasal cavity, which may lead to sudden nosebleed when sleeping unconsciously or rubbing or picking the nose. 2. Hypertension: when the blood pressure is elevated or poorly controlled, it may lead to the damage of blood vessels in the whole body, which may cause sudden nosebleed when the blood vessels of nasal mucosa are damaged. 3. Nasopharyngeal cancer: Nasopharyngeal cancer patients’ nosebleed is mainly due to the invasion of cancerous lesions into nasal mucosa or invasion of blood vessels, which leads to bleeding, and the amount of nosebleed is obviously different according to the degree of mucous membrane damage or the size of invaded blood vessels. Sudden nosebleed in sleep may also be caused by dry rhinitis, nasal capillary hemangioma, etc. Therefore, if you have frequent nosebleed, you should go to ENT department for professional examination in time, so as to clarify the cause of the disease, and then treat it according to the cause.