The first part of the chapter describes the activating effect of sleep on epileptic syndromes and underlying epileptic networks. The most important sleep-activated idiopathic epileptic disorders are addressed according to their vigilance-related symptomatology, underlying circuits, and the association of sleep activation and the degree and type of cognitive impairment. In the light of recent sleep research data, sleep activation of epileptic discharges has been shown to interfere with the physiological functions of nonrapid eye movement (NREM) sleep, explaining how epilepsy exerts an important influence on cognitive function, mainly during developmental windows. According to the role of sleep in cognitive impairment, epilepsies can be divided into two groups: (1) those with the involvement of the thalamocortical system, in which cognitive impairment via interference of sleep function is important; and (2) those in which the thalamocortical system is not a key factor in determining how epilepsy may induce cognitive impairment. The second part of the chapter is devoted to the role of sleep activation as a diagnostic tool.