THE ECHIDNA TACHYGLOSSUS-ACULEATUS COMBINES REM AND NON-REM ASPECTS IN A SINGLE SLEEP STATE - IMPLICATIONS FOR THE EVOLUTION OF SLEEP

Placental and marsupial mammals exist in three states of consciousness : waking, non-REM sleep, and REM sleep. We now report that the echidna Tachyglossus aculeatus, a representative of the earliest branch of ma mmalian evolution (the monotremes), does not have the pattern of neuro nal activity of either of the sleep states seen in nonmonotreme mammal s. Echidna sleep was characterized by increased brainstem unit dischar ge variability, as in REM sleep. However, the discharge rate decreased and the EEG was synchronized, as in non-REM sleep. Our results sugges t that REM and non-REM sleep evolved as a differentiation of a single, phylogenetically older sleep state. We hypothesize that the physiolog ical changes that occur during postnatal sleep development parallel ce rtain aspects of the changes that have occurred during the evolution o f sleep-waking states in mammals.