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Sleep in marine mammals

All terrestrial mammals show relatively high-voltage neocortical EEG
activity bilaterally during NREM sleep. By contrast, cetaceans (whales
and dolphins) almost never have high-voltage slow waves in both
hemispheres at the same time. Manatees
(Trichechus inunguis, a member of the order Sirenia) also have unihemispheric
slow waves. In all marine mammals studied to date, the
eye contralateral to the brain hemisphere with slow waves is almost
always closed while the other eye is almost always open. There have
been no published reports documenting REM sleep in cetaceans, making
them the only studied mammals in which this state has not been
observed.
The bottlenose dolphin (Tursiops truncatus), when not floating or
resting on the bottom, generally swims in a single direction (usually
counterclockwise) even as the hemisphere with slow waves alternates.
Some smaller cetacean species are rarely, if ever, immobile. They move
and avoid obstacles 24 hours a day from birth until death, even during
unihemispheric slow-wave activity. These animals may never exhibit
the immobility that we use in terrestrial mammals to define the state
of sleep.
Slow (high-voltage 1–4-Hz) waves and spindle (high-voltage 8–12
Hz) waves are not by themselves conclusive evidence for sleep as
defined at the beginning of this review. They have been linked to sleep
because they generally accompany the behavioural signs of sleep in
terrestrial mammals. However, even in terrestrial mammals, certain
drugs can induce bilateral high-voltage EEG activity in individuals that
are clearly awake. Moreover, high-voltage neocortical activity is not
a required indicator of sleep; a NREM sleep state with low-voltage neocortical
activity has been documented in rodents. REM sleep is characterized
by a low-voltage EEG, indistinguishable from that of waking
in many animals.
Studies of arousal threshold have not been performed on cetaceans
across putative sleep–wake cycles. Only one study has looked for sleep
rebound after EEG slow waves were prevented by disturbing cetaceans.
Although some evidence for rebound was seen, the response was
highly variable, with some animals showing little or no recovery of lost
slow waves. The amount of slow waves after deprivation bore no consistent
relation to the amount of slow-wave activity that had been lost.
Neuronal recording studies in terrestrial mammals suggest that the
axial movements occurring in cetaceans during both normal swimming
and swimming with unihemispheric slow waves are accompanied
by activation of large regions of the brainstem reticular formation.
This is quite different from the great reduction in brainstem reticular
activity that characterizes NREM sleep in terrestrial mammals.
Cetaceans deftly avoid obstacles during this constant motion, sug-
gesting accurate bilateral processing of sensory information.
If sensory and motor systems do not show typical sleep inactivation,
if behavioural and sleep rebound evidence for sleep debt is weak,
do cetaceans sleep as conventionally defined? Certainly further work
is necessary to determine which, if any, neurochemical and neurophysiological
aspects of sleep, other than unilateral neocortical slow
waves and eye closure, are preserved in cetaceans and might constitute
the ‘essence’ of sleep. Alternatively, we may need to revise our assumption
that sleep is fundamentally similar in cetaceans and terrestrial
mammals and focus on how cetaceans can dispense with some of the
most readily detectable aspects of sleep.

NATURE|Vol 437|27 October 2005 - "Clues to the functions of mammalian sleep"

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Some smaller cetacean species are rarely, if ever, immobile. They move and avoid obstacles 24 hours a day from birth until death, even during unihemispheric slow-wave activity

Slow (high-voltage 1–4-Hz) waves and spindle (high-voltage 8–12 Hz) waves are not by themselves conclusive evidence for sleep as defined at the beginning of this review. They have been linked to sleep because they generally accompany the behavioural signs of sleep in
terrestrial mammals. However, even in terrestrial mammals, certain drugs can induce bilateral high-voltage EEG activity in individuals that are clearly awake. Moreover, high-voltage neocortical activity is not a required indicator of sleep; a NREM sleep state with low-voltage neocortical
activity has been documented in rodents35. REM sleep is characterized by a low-voltage EEG, indistinguishable from that of waking in many animals.

Most theories suggest a role for non-rapid eye movement (NREM) sleep in energy conservation and in nervous system recuperation. Theories of REM sleep have suggested a role for this state in periodic brain activation during sleep, in localized recuperative processes and in emotional regulation.