A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. Headaches can result from a wide range of causes both benign and more serious. Brain tissue itself is not sensitive to pain as it lacks pain receptors. Rather, the pain is caused by disturbance of the pain-sensitive structures around the brain. Nine areas of the head and neck have these pain-sensitive structures, which are the cranium (the periosteum of the skull), muscles, nerves, arteries and veins, subcutaneous tissues, eyes, ears, sinuses and mucous membranes. There are a number of different classification systems for headaches. The most well-recognized is that of the International Headache Society. Headache is a non-specific symptom, which means that it has many possible causes, including fatigue and sleep deprivation, stress, the effects of medications and recreational drugs, viral infections and common colds, head injury, rapid ingestion of a very cold food or beverage, dental or sinus issues, and many more. Treatment of a headache depends on the underlying cause, but commonly involves pain killers. Some form of headache is one of the most commonly experienced of all physical discomforts.