mammals have wide diversity in sleep phenomena. generally, they go through periods of alternating non-rem and rem sleep, but these manifest differently. horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for rem sleep (which causes muscular atony) for short periods. giraffes, for example, only need to lie down for rem sleep for a few minutes at a time. bats sleep while hanging upside down. male armadillos get erections during non-rem sleep, and the inverse is true in rats. early mammals engaged in polyphasic sleep, dividing sleep into multiple bouts per day. higher daily sleep quotas and shorter sleep cycles in polyphasic species as compared to monophasic species, suggest that polyphasic sleep may be a less efficient means of attaining sleep’s benefits. small species with higher bmr may therefore have less efficient sleep patterns. it follows that the evolution of monophasic sleep may hitherto be an unknown advantage of evolving larger mammalian body sizes and therefore lower bmr.