Hippocampal EEG theta power density is similar during slow-wave sleep and paradoxical sleep. A long-term study in rats.

Rat hippocampal EEG and motor activity were studied during 15 days in relation to the vigilance state and to the light-dark cycle with a computerized system. During slow-wave sleep (SS) the hippocampal EEG has an outstanding mean power density in the theta band, similar to the large values present during paradoxical sleep. A circadian modulation was found for motor activity in SS and wakefulness (i.e. day-sleep is more restless, day-wakefulness is more quiet than night), and for EEG mean theta power in SS (i.e. less during day-sleep than night-sleep). These data underline the importance of analyzing the dark period when studying nocturnal animals.