 Since monotremes (egg-laying mammals) are considered to represent one of the evolutionarily oldest groups of mammals, they have been subject to special interest in the study of mammalian sleep.As early studies of these animals could not find clear evidence for REM sleep, it was initially assumed that such sleep did not exist in monotremes, but developed after the monotremes branched off from the rest of the mammalian evolutionary line, and became a separate, distinct group.However, EEG recordings of the brain stem in monotremes show a firing pattern that is quite similar to the patterns seen in REM sleep in higher mammals.