Phylogeny and the function of REM sleep.

Phylogenetic studies in placental and marsupial mammals have demonstrated three major correlates of increased REM sleep time across these species. These are high amounts of non-REM sleep time, safe sleep conditions and immaturity at birth. While these variables explain approximately 30% of the variance in REM sleep time across these orders, these relations are violated when animals other than placentals are included. Birds are small, many have safe sleeping situations and are certainly immature at birth, yet they have less REM sleep than the vast majority of mammals. The echidna is immature at birth, has high amounts of non-REM sleep and safe sleeping conditions, yet has been reported to have no REM sleep. Our recent studies in the echidna indicate that REM and non-REM sleep did not evolve sequentially, but rather evolved as a differentiation of a primitive state which held the seeds of both sleep states. The echidna sleeps with an activated brainstem and EEG synchronized forebrain. Future studies of sleep phylogeny need to compare the behavior of key neuronal groups across the sleep cycle, since these results indicate that EEG variables and sleep state durations may given an inadequate picture of the nature of brain activity during sleep.