The relationship between event-related potentials (ERPs) and sleep alterations in patients with sleep disorders is still controversial. Whether the ERP alterations are affected by sleep loss and sleep discontinuity can be addressed by studying presleep-to-postsleep changes in ERPs. We investigated if presleep-to-postsleep ERP latencies and amplitudes differ in patients with insomnia and sleep-related breathing disorders and whether these changes are affected by the degree of subjective and objective sleep alterations.