Nosebleeds at night when a four-year-old baby sleeps may be caused by dry air, nasal mucous membrane erosion, allergic rhinitis or thrombocytopenia. 1. four-year-old baby sleep at night if the air conditioning or winter climate dry, easy to lead to the nasal mucosa dry, brittle increase, coupled with the night to sleep to drink less water, which is prone to the phenomenon of night sleep nosebleed. 2. If a four-year-old baby has the bad habit of picking his nose often, or the nose has been traumatized, which can easily lead to nasal mucous membrane erosion, if you rub your nose at night when you sleep, it is easy to cause the erosion of the bleeding phenomenon. 3. If the baby suffers from allergic rhinitis disease, the nasal mucosa capillary fragility increases, the nose is easy to cause nosebleed phenomenon when sleeping at night after stimulation. 4. If the baby suffers from thrombocytopenia, due to bleeding abnormalities in the coagulation function, after rubbing the nose will also appear the phenomenon of nosebleeds. The four-year-old baby sleeps at night nosebleed, there may be other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.