•The sleep state is dynamic, and various sleep stages have sleep influence seizures in different ways that have clinical importance.•Interictal discharges are more commonly seen during sleep.•Initiation and propagation of seizures can change depending on sleep state.•Anticonvulsant drugs can modulate sleep itself, and also have influences on sleep disorder. Sleep is a dynamic process, during which the electrical rhythms of the brain orchestrate a complicated progression of changing frequencies, patterns and connectivity. Each stage of sleep is different electrophysiologically from wakefulness, and from other sleep stages. It should be no surprise, then, that the various sleep states influence the origin, suppression, and spread of seizures, and that different seizure types are affected in individual (and sometimes contradictory) ways. While much of the electrical symphony that occurs in both normal and epileptic brains is incompletely understood, at the basic level some interesting and often clinically important influences of the various sleep states have been identified. While interictal epileptiform activity is not a seizure, these markers of epilepsy are affected by sleep. Both initiation and propagation of various seizure types are affected by sleep, and these are discussed separately. Finally, the relationship between sleep and epilepsy is clearly reciprocal, and the final sections will explore the changes in sleep that seizures and antiepileptic drugs can induce.