We studied the effect of social stress on sleep electroencephalogram (EEG) in rats. Animals were subjected to a single social defeat by introducing them in the cage of an aggressive male conspecific for 1 h. The animals responded to the social conflict by a sharp increase in EEG slow-wave activity (SWA) during non-rapid eye movement sleep (NREM) afterwards. Since SWA has been identified as an indicator of sleep intensity, the results suggest that acute stressors may accelerate the build up of sleep debt. Sleep intensity may, thus, not only depend on the duration of prior wakefulness but also on the nature of the waking experience. The strong increase in SWA after social defeat indicates that sleep may function to offset the mental loads imposed on the nervous system during wakefulness.