Patients who wake up in pain while sleeping because of a severe headache may have a cluster headache attack. Cluster headaches are painful in one eye and around the orbital area, most often in the evening, and develop into a severe swelling or drilling pain in the head within a few minutes. It may be accompanied by conjunctival congestion and tearing in one eye, and in some cases, vomiting. Patients may have multiple episodes of these symptoms in a short period of time, at approximately the same time each day, as if there is a fixed clock. Patients may also wake up with headaches in their sleep, which can be caused by serious diseases such as subarachnoid hemorrhage or cerebral hemorrhage. For example, subarachnoid hemorrhage can cause patients to have very severe headaches, usually presenting a blast-like pain that is unbearable for the patient.