This review describes empirical evidence for a bidirectional relationship between tension-type headache (TTH) and sleep. In its most severe form, chronic TTH (CTTH) affects 2-3 % of the population and can be very disabling. Sleep dysregulation triggers episodic TTH, and sleep disorders may complicate and exacerbate headache. The majority of CTTH sufferers also have insomnia, and longitudinal data suggest that insomnia is a risk factor for new-onset TTH. Similarly, observational studies suggest that sleep disturbance is a risk factor for new-onset TTH and for progression from episodic to chronic TTH (i.e., headache "chronification"). CTTH is the most common headache secondary to sleep apnea and other sleep-related breathing disorders. Psychiatric disorders are comorbid with both TTH and insomnia and may further complicate diagnosis and treatment. Developments in diagnostic classification of sleep-related headache are presented.