Is there REM sleep in reptiles? A key question, but still unanswered

REM sleep was initially described in mammals and was later found in birds. As the results obtained in reptiles were unconvincing, this suggested for years that only homeothermic species have REM sleep. However, recent studies in lizards also suggest the presence of two electrophysiological states and provide a better picture of their sleep. Nevertheless, due to differences in the expression of these states in two lizard species, as well as missing information, homology with mammalian REM sleep still could not be established. As ectotherms and homeotherms have very different physiology and neuroanatomy, we expect that, even if a state like REM sleep exists in ectotherms, its phenotypic expression should be different. Therefore, we think that a comprehensive understanding of the evolution of REM sleep will come from efforts to examine both the similarities and difference between sleep states in homeotherms and ectotherms.