Why REM Sleep is Reduced in Aquatic and Semi-aquatic Mammals? A Discussion of the Possible Theories

Abstract

Although most mammals experience rapid eye movement (REM) sleep, the primary function of REM sleep is still debatable. Current data show that aquatic mammals have absent or reduced REM sleep and that semi-aquatic mammals have reduced REM sleep when in water. A few theories have been proposed to explain this reduction in REM sleep in aquatic mammals. Some investigators proposed that seals’ switch from bilateral to uni-hemispheric non-REM sleep in water may render REM sleep unnecessary, and suggested that REM may serve to reverse the brain cooling that occurs in bilateral non-REM sleep. Another proposed theory is the effect of gravity on REM sleep and that aquatic and semi-aquatic species do not need REM sleep in low weightless environments due to the effect of gravity on sleep. In this viewpoint, we critically review a few proposed theories and their supporting evidence.