Sleep is suggested to be crucial for the processing and storage of new information. Several learning tasks have been shown to increase the amount of rapid eye movement sleep (REMS) with its typical theta activity (6-8 Hz) relative to total sleep time. Vice versa, REMS deprivation is able to affect memory consolidation following some, but not all learning tasks. Furthermore, recent studies have shown an increase of spindle activity (12-15 Hz) within the electroencephalogram (EEG) of nonREMS as well. The enhancement of both spindle and theta activity is suggested to serve as background activity for the synchronization of those neuronal pathways that were involved in the registration and, later on, participate in the long-term storage of new information in defined brain regions. In the present study, the presentation of a novel object to rats enhanced the amount of preREMS, an intermediate sleep stage with high spindle activity, within the first 2 h of the subsequent sleeping phase. Four hours later, the amount of REMS was increased as well. However, there were no changes in the EEG power spectra of nonREMS, preREMS and REMS. We therefore hypothesize that the increase of preREMS and REMS amounts and the related spindle and theta activity stand for the processing and storage of new information about the presented novel objects.