For cetaceans, unihemispheric sleep is the only way to sleep, while in seals and birds, unihemispheric sleep events are intermingled with bihemispheric and rapid eye movement sleep events. The neural mechanisms promoting wakefulness dominate one side of the brain, while those promoting sleep predominates the other side. The neural mechanisms of unihemispheric sleep are unknown, but assuming that the neural structures involved in sleep in cetaceans, seals, and birds are similar to those of terrestrial mammals, it is suggested that they involve the interaction of structures of the hypothalamus, basal forebrain, and brain stem. However, the behavior and health of aquatic mammals and birds does not seem at all impaired by the reduction of sleep. Compared to bihemispheric sleep, unihemispheric sleep would mean a reduction of the time spent sleeping and of the associated recovery processes. In birds, antipredation vigilance is the main function of unihemispheric sleep, but in domestic chicks, it is also associated with brain lateralization or dominance in the control of behavior. In dolphins, eared seals, and manatees, unihemispheric sleep allows them to have the benefits of sleep, breathing, thermoregulation, and vigilance. However, certain marine mammals and species of birds show a different sleep behavior, in which one cerebral hemisphere sleeps while the other is awake. In addition, sleep is a periodically necessary behavior and therefore, in the majority of animals, it involves the whole brain and body. Sleep is a behavior characterized by a typical body posture, both eyes’ closure, raised sensory threshold, distinctive electrographic signs, and a marked decrease of motor activity.
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