The relationship between sleep and headache has been known for over a century. Headache and sleeping problems are both some of the most commonly reported problems in clinical practice, and cause considerable social and family problems, as well as socio-economic impact and costs. There is a clear association between headache and sleep disturbances, especially headaches occurring during the night or early morning. The mechanism and causes are complex, multifactorial and poorly understood. Headache disorders like migraine, tension-type headache, cluster headache and hypnic headache all affect or are directly related to sleep disturbances and daytime functioning. Sleep fragmentation, insomnia and hypersomnia all show relations to headache. Primary sleep disorders like insomnia, hypersomnias including sleep disordered breathing are all associated with and may cause headache. Furthermore medical, psychiatric and rheumatic diseases are associated with sleep disturbances and headache. The current knowledge about headache and sleep is still sparse and further research is advocated. 2002 Harcourt Publishers Ltd