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It actually happened to me too when i was younger. Nosebleeds can occur spontaneously when the nasal membranes dry out and crack. This is common in dry climates, or during the winter months when the air is dry and warm from household heaters. People are more susceptible to a bloody nose if they are taking medications which prevent normal blood clotting warfarin (Coumadin), aspirin, or any anti-inflammatory medication]. In this situation, even a minor trauma could result in significant bleeding.

When it bleeds just sleep upset down i.e sleep making your nose up. Better consult to the doctor.

Are you around to much heat? If so that could be the cause

go to the hospital you're losing a lot of blood