Cluster headaches refer to headaches that occur in batches, or clusters, as often as several times a day for several days, weeks, or months at a time. They are most common in adults, but occasionally occur in children. These headaches are characterized by severe pain behind one eye, which often becomes red and watery. The nostril on the same side may become stuffy and runny. Headaches can last from 20 minutes to an hour and will often begin in the middle of the night, a few hours after falling asleep. Children with severe headaches, should have a complete physical examination to pinpoint the source of the pain and receive the appropriate treatment.