Fur seals display a strong drive for bilateral slow-wave sleep while on land.

Fur seals (pinnipeds of the family Otariidae) display two fundamentally different patterns of sleep: bilaterally symmetrical slow-wave sleep (BSWS) as seen in terrestrial mammals and slow-wave sleep (SWS) with a striking interhemispheric EEG asymmetry (asymmetrical SWS or ASWS) as observed in cetaceans. We examined the effect of preventing fur seals from sleeping in BSWS on their pattern of sleep. Four northern fur seals (Callorhinus ursinus) kept on land were sleep deprived (SD) of BSWS for 3 consecutive days, followed by 1 recovery day. EEG asymmetry was evaluated both visually and by EEG spectral analysis. SD significantly reduced the percentage of high-voltage BSWS (on average to 14% of baseline) and REM sleep (to 60% of baseline) whereas the percentage of low-voltage BSWS was not affected. During the SD period, all seals repeatedly tried to enter BSWS (109-411 attempts per day). SD significantly increased the amount of ASWS in each seal when scored visually (to 116-235% of baseline) and the difference in the EEG slow-wave activity (spectral power in the range of 1.2-4.0 Hz) between the two hemispheres (117-197%) as measured by the asymmetry index. High-voltage BSWS and the amount of SWS in each hemisphere were significantly elevated during the first 4 h of recovery. These data indicate that fur seals display a homeostatic response to the loss of SWS and that alternating SWS in the two hemispheres does not adequately compensate for the absence of BSWS.