In the last few decades, several works on cognitive processing during sleep have emerged. The study of cognitive processing with event related potentials (ERPs) during sleep is a topic of great interest, since ERPs allow the study of stimulation with passive paradigms (without conscious response or behavioural response), opening multiple research possibilities during different sleep phases. We review ERPs modulated by cognitive processes during sleep: N1, Mismatch Negativity (MMN), P2, P3, N400-like, N300-N550, among others. The review shows that there are different cognitive discriminations during sleep related to the frequency, intensity, duration, saliency, novelty, proportion of appearance, meaning, and even sentential integration of stimuli. The fascinating results of cognitive processing during sleep imply serious challenges for cognitive models. The studies of ERPs, together with techniques of neuroimaging, have demonstrated the existence of cognitive processing during sleep. A fundamental question to be considered is if these cognitive phenomena are similar to processing that occurs during wakefulness. Based on this question we discussed the existence of possible mechanisms associated with sleep, as well as the specific cognitive and neurophysiologic differences of wakefulness and sleep. Much knowledge is still required to even understand the conjunction of dramatic changes in cerebral dynamics and the occurrence of cognitive processes. We propose some insights based on ERPs research for further construction of theoretical models for integrating both cognitive processing and specific brain sleep dynamics.