Ultradian rhythm of 100 min in the dark phase EEG of the rat.

Using the "variance in statistics" as an index of electroencephalogram (EEG) parameters, we observed the cortico-hippocampal EEG rhythm under a 12:12-h light-dark condition in the rat with chronically implanted electrodes for EEG recording. The above EEG variance was simply measured in real time and on line through a personal computer. It corresponded to EEG slow wave activity and expressed the process of slow wave sleep as described in the two-process model by Borbély et al. Only in the dark phase, mean power spectral density of the EEG variance had a significant peak at about 1/100 cycles/min. This 100-min rhythmicity similar to the basic rest-activity cycle in human beings was observed in rats, particularly in the dark (active) phase for nocturnal animals. We propose that this ultradian 100-min rhythm is essential for the rat to maintain the waking state dominantly over the 12-h dark period.