The May issue of Scientific American had a small article on the abilityJoe, actually some mammals have unihemispheric sleeping. Many marine mammals, including bottle-nosed doplphins, engage only in unihemispheric non-rapid-eye-movement sleep. Dolphins don't have REM sleep because its associated skeletal muscle hypotonia would cause drowning. If unihemispheric sleep developed to prevent predation in birds, I would expect it to have evolved in heavily preyed upon mammals also, which I don't believe it has. Would unihemispheric sleep in birds be more of an adaption to prevent falling during perching?With regards to dinosaurs: Early studies suggested that montremes did not have REM sleep, indicating REM sleep started after the start of the mammalian line. However more recent work demonstrates that the platypus not only has REM sleep but has more of it than any other animal, suggesting REM sleep began early in vetebrate evolution. Does anyone know about REM sleep in crocodylians and birds? In short, did dinosaurs dream?--Ken Clay, M.D.
of birds to keep one eye open while asleep, known as unihemispheric sleeping.
One side slept while the other side stayed on guard... it probably developed to guard against predators, a sensible notion. They suggested that mammals didn't develop this behavior because early mammals likely slept in safe burrows.