Simultaneous coexistence of unihemispheric sleep and wakefulness has been observed in marine mammals. Similarly, asymmetric sleep has been found also in birds. Besides, in both cases the inter-hemispheric asymmetry is not constant and after a while, the dominant hemisphere turns non-dominant. However, the constancy in inter-hemispheric dominance is generally accepted during sleep in most mammals. This report extends to rats the inter-hemispheric dominance switching during NREM, REM and wakefulness.

Five male Wistar rats, individually housed and maintained under normal laboratory conditions were surgically prepared for conventional polysomnography with symmetric electrodes in the frontal and parietal cortex. After recovery, the animals were submitted to 48h of continuous recording. FFT analysis was performed on several 30s segments of EEG selected from each animal and corresponding to unambiguous Waking, NREM and REM. First, it was observed that the EEG of rats lacks right-left population asymmetry. In consequence, the EEGs were grouped according to preferred and non-preferred depending on the hemisphere with maximal power. In this way a robust asymmetry was found in all animals during one half of the photoperiod. Then, a significant switching in inter-hemispheric power was found in all bands during the second half of the photoperiod. Interestingly, a significant inter-hemispheric switching was found in alpha and theta power during waking, in delta during NREM and in theta during REM. In conclusion, the existence of asymmetric sleep should be considered a general feature of mammals.