Tool Module: Sleep in Other Animals
Not all animals experience sleep the same way we humans do each night. Like many
other phenomena, sleep has grown more complex in the course of evolution. For
example, reptiles show many signs of non-REM sleep, but do not seem to experience
Animals less highly evolved than reptiles (such as amphibians,
fish, insects, and molluscs) go through quiescent periods that superficially resemble
sleep, but scientists are not sure whether these are in fact ancestral forms of
sleep, or rather simply forms of resting that are peculiar to these species.
Sleep in its most evolved form is thought to have appeared first among birds,
about 100 million years ago. But sleep in birds is still very different from sleep
in humans. In contrast, mammals such as rats, cats, and monkeys have a form of
sleep whose various stages are far more similar to our own.
there is a general tendency for large predators, such as the great cats, to spend
a higher proportion of their sleeping time in deep non-REM sleep, whereas their
prey spend a higher proportion of theirs in the lighter stages of non-REM sleep.
Prey animals also generally experience very little REM sleep; the associated paralysis
would render them highly vulnerable to their predators.
Size also seems
to affect the duration of sleep. Small mammals generally tend to sleep longer
and to experience more REM sleep than larger mammals do.
are still relatively immature at birth, such as opossums (or humans!) also generally
tend to get more REM sleep than animals that can walk or eat independently very
shortly after they are born. This high proportion of REM sleep in species with
vulnerable offspring (as much as 25% in many species) persists into adulthood.
Compared with humans, who sleep an average of 7 or 8 hours per night, there
are a number of big sleepers in the animal kingdom. Bats spend nearly 20 hours
of every day asleep! For opossums and pythons, the figure is 18 hours. Cats are
pretty good sleepers too, averaging 11 to 14 hours of sleep per day.
The survival of prey animals is critically dependent on their maintaining
a state of permanent alertness. Hence species such as rabbits and giraffes sleep
only for brief intervals, with giraffes rarely totalling more than 2 hours per
This extreme variability in the amount of time that animals
sleep obviously raises many questions about the role of sleep in various species.
It also shows what an important influence the genetic inheritance of a species
has on its sleep patterns. One thing is certain: all species must sleep at least
a few hours per day, so sleep seems to have some crucial function that would otherwise
have disappeared among some species in the course of evolution.
The sleep of certain animals has many other amazing characteristics. In some
species, such as dolphins, which must return to the water’s surface regularly
to breathe, only half of the brain sleeps at a time. While one brain hemisphere
displays all the typical signs of non-REM sleep, the other remains awake. Since
returning to the surface to breathe is a voluntary act for these animals, this
special ability of their brain enables them to perform two vital functions at
once: sleeping and breathing. The dolphin’s sleep pattern is as follows:
2 hours with one hemisphere asleep, then 1 hour with both hemispheres awake, then
2 hours with the other hemisphere asleep, then 1 hour with both hemispheres awake,
and so on, for 12 hours every night.
One species of blind dolphin that
lives in the turbid waters of the Indus River in Pakistan cannot afford to spend
such long periods with half of its brain asleep. Instead, it manages to catch
“micro-naps” that last only 4 to 6 seconds, but added up, they
nevertheless amount to 7 hours of sleep per day!
Other animals, such
as horses and elephants, can sleep standing up, thanks to a system that lets them
lock their knees. Bats sleep hanging upside down. And lastly, some birds, such
as pigeons, while remaining asleep, periodically open their eyes to keep a watch
on their surroundings.