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. in fact, the largest amount of rem sleep known in any animal is found in the platypus. rem electrical activation does not extend at all to the forebrain in platypods, suggesting that they do not dream. the average sleep time of the platypus in a 24-hour period is said to be as long as 14 hours, though this may be because of their high-calorie crustacean diet.