Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep

Several animals mitigate the fundamental conflict between sleep and wakeful ness by engaging in unihemispheric sleep, a unique state during which one c erebral hemisphere sleeps while the other remains awake. Among mammals, uni hemispheric sleep is restricted to aquatic species (Cetaceans, cared seals and manatees). in contrast to mammals, unihemispheric sleep is widespread i n birds, and may even occur in reptiles. Unihemispheric sleep allows surfac ing to breathe in aquatic mammals and predator detection in birds. Despite the apparent utility in being able to sleep unihemispherically, very few ma mmals sleep in this manner. This is particularly interesting since the rept ilian ancestors to mammals may have slept unihemispherically. The relative absence of unihemispheric sleep in mammals suggests that a trade off exists between unihemispheric sleep and other adaptive brain functions occurring during sleep or wakefulness. Presumably, the benefits of sleeping unihemisp herically only outweigh the costs under extreme circumstances such as sleep ing at sea. Ultimately, a greater understanding of the reasons for little u nihemispheric sleep in mammals promises to provide insight into the functio ns of sleep, in general. (C) 2000 Elsevier Science Ltd. All lights reserved .