Sleep in the platypus

We have conducted the first study of sleep in the platypus Ornithorhynchus anatinus. Periods of quiet sleep, characterized by raised arousal threshold s, elevated electroencephalogram amplitude and motor and autonomic quiescen ce, occupied 6-8 h/day. The platypus also had rapid eye movement sleep as d efined by atonia with rapid eye movements, twitching and the electrocardiog ram pattern of rapid eye movement. However, this state occurred while the e lectroencephalogram was moderate or high in voltage, as in non-rapid eye mo vement sleep in adult and marsupial mammals. This suggests that the low-vol tage electroencephalogram is a more recently evolved feature of mammalian r apid eye movement sleep. Rapid eye movement sleep occupied 5.8-8 h/day in t he platypus, more than in any other animal. Our findings indicate that rapi d eye movement sleep may have been present in large amounts in the first ma mmals and suggest that it may have evolved in pre-mammalian reptiles. (C) 1 999 IBRO. Published by Elsevier Science Ltd.