There is an intricate reciprocal relationship between epilepsy and sleep. The seizure threshold is often affected by changes in the level of arousal; certain seizure types occur predominantly or almost exclusively during sleep or upon awakening; many epileptiform electroencephalogram abnormalities are activated by sleep or sleep deprivation. Inversely, certain epilepsies are often associated with sleep disturbances, and epilepsy can affect sleep patterns and sleep architecture. Also, it may be difficult to differentiate certain nocturnal nonepileptic events from epileptic seizures occurring during sleep. Finally, antiepileptic drugs used in the treatment of sleep-related epilepsies can have an effect on sleep. The following is an analysis and review of these complex interactions between epilepsy and sleep.