Headache is a common disorder in the general population. Fasting headache is coded to Group 10 of the second edition of the International Classification of Headache Disorders ("Headache attributed to disorder of homeostasis"). A study conducted in Denmark's general population found a lifetime prevalence rate of 4.1% for fasting headache. Fasting headache is usually diffuse or located in the frontal region, and the pain is nonpulsating and of mild or moderate intensity. In most cases, the headache occurs after at least 16 h of fasting and resolves within 72 h after resumption of food intake. The likelihood of developing fasting headache increases directly with the duration of the fast. Headache sufferers have a higher risk of developing headache during fasting than people who do not usually suffer from headache. Hypoglycemia and caffeine withdrawal have been especially implicated as causative factors, but much remains to be understood about this topic.