The different animal species exhibit a vast range of sleep experiences.
A number of factors influence their patterns, including their size, metabolic
rate, and whether they are predators or prey.
In mammals, the alternation of slow-wave sleep
and REM (with animals it is usually termed paradoxical) sleep occurs as
in humans - but a curious anomaly has been found to be the Australian spiny
ant-eater, which seems to show only slow wave sleep.
Grazing animals tend to have little sleep,
as do those which sleep in the open or spend a long time feeding. As a
general rule, animals which are preyed upon have less paradoxical sleep,
while larger animals have more slow wave sleep.
Cats may sleep for up to 16 hours a day. They
enter paradoxical sleep roughly every 30 minutes after sleep onset. In
that condition, their neck muscles are fully relaxed and slight twitching
is sometimes observed.
Rabbits do not exhibit muscular paralysis
during paradoxical sleep. Their ears are flat though during that period.
Horses may sleep only 5 hours a day - standing 22 hours and having 45 minutes
of paradoxical sleep and 2 hours of slow wave sleep.
Dolphins, strangely, shut down one side of
their brain alternately in sleep. The animal never stops moving and there
is no muscular relaxation as in humans and no measurable paradoxical sleep.
Probably the arrangement is to permit the dolphin to maintain an awareness
and to surface for air occasionally.
Birds also show an alternating cycle between
slow wave sleep and paradoxical sleep. Muscle tone is not lost completely,
because they need to roost and maintain posture. Some birds also close
down a hemisphere of the brain alternately in sleep.
We don't yet know what animals experience
during sleep, but anyone who has seen dogs making slight movements and
sounds in sleep is bound to assume that the animal is experiencing a vivid
dream. And so it may be for many varied species.