The existence of a relationship between epilepsy and sleep has been realized since ancient times. Despite extensive clinical observations and experimental investigations, the precise nature of this relationship however remains unclear and the mechanisms responsible for this are poorly understood. In this study, the authors review the neurophysiology of sleep, concentrating on the various oscillations and electrographic activities associated with different stages of sleep. The authors also review how sleep affects both epileptic seizures and interictal epileptiform discharges. Finally, they discuss the mechanisms, both confirmed and theoretical, which are thought to be responsible for the effect of sleep on epileptic seizures and epileptiform discharges.