Cetacean sleep: An unusual form of mammalian sleep

Abstract

Our knowledge of the form of lateralized sleep behavior, known as unihemispheric slow wave sleep (USWS), seen in all members of the order Cetacea examined to date, is described. We trace the discovery of this phenotypically unusual form of mammalian sleep and highlight specific aspects that are different from sleep in terrestrial mammals. We find that for cetaceans sleep is characterized by USWS, a negligible amount or complete absence of rapid eye movement (REM) sleep, and a varying degree of movement during sleep associated with body size, and an asymmetrical eye state. We then compare the anatomy of the mammalian somnogenic system with what is known in cetaceans, highlighting areas where additional knowledge is needed to understand cetacean sleep. Three suggested functions of USWS (facilitation of movement, more efficient sensory processing and control of breathing) are discussed. Lastly, the possible selection pressures leading to this form of sleep are examined, leading us to the suggestion that the selection pressure necessitating the evolution of cetacean sleep was most likely the need to offset heat loss to the water from birth and throughout life. Aspects such as sentinel functions and breathing are likely to be proximate evolutionary phenomenon of this form of sleep.

First evidence of a 200-day non-stop flight in a bird

“…All three individuals provided very similar patterns, but some differences in absolute values. During the breeding period, Alpine swifts are known to spend the nights roosting in the colony often close to their nest 17 . The clearly twofold distribution of pitch and activity ( Fig.…”

“…Their activity pattern reveals that they can stay airborne continuously throughout their non-breeding period in Africa and must be able to recover while airborne. To date, such long-lasting locomotive activities had been reported only for animals living in the sea 16,17 .…”

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

“…This is a form of NREM sleep in which the state of being asleep alternates between two brain hemispheres and is accompanied by only one closed eye at a time, the one that is contralateral to (and is innervated by) the sleeping hemisphere. 24 The likely adaptive nature of USW sleep stems from the ability of, for example, a fur seal to monitor its immediate environment for predators and conspecifics using a single open eye, to continue to swim, and to continue staying ''halfway'' alert most of the time. Among terrestrial animals, only birds exhibit aspects of unihemispheric sleep, but not as pronounced as in cetaceans.…”

“…Among terrestrial animals, only birds exhibit aspects of unihemispheric sleep, but not as pronounced as in cetaceans. 24,258 The USW sleep has evolved in marine mammals presumably because it is particularly adaptive in aquatic settings, as the terrestrial environment can provide an animal with a measure of protection during sleep, for example, in burrows and trees. In addition, vulnerable grazing ungulates, such as antelopes, tend to sleep in herds, in which there are always some animals (sentinels) that are not asleep.…”

“…In contrast, when a fur seal sleeps in water, its EEG patterns indicate a unihemispheric slow wave (USW) sleep, as in whales and dolphins. 24,259 This switch from BSW sleep on land to USW sleep in water may be an opportunity to address a prediction of the FG hypothesis. In the BSW sleep (on land), when both hemispheres sleep at the same time, specific (e.g., relevant to sleep causation) protein fragments in the two hemispheres of the seal's brain would be expected to be present at equal levels, irrespective of the timing of brain analyses during sleep-wakefulness cycles.…”