What is sleep?

Sleep is a naturally occurring state of deep rest.  In humans, sleep is characterized by disengagement of our conscious minds, and by inactivity of our muscles.   All mammals, and most animals – including birds, reptiles, fish, even insects – need some form of sleep to survive. 

The amount of sleep, when it occurs, and how it happens varies widely among different species.  A brown bat, for instance, sleeps about 83% of its life, while a giraffe sleeps less than 2 hours a day.


Total Sleep Requirements for Various Species

Species Average Total Sleep Time (hours/day)

brown bat  19.9

python  18.0

owl monkey  17.0

human infant 16.0

tiger  15.8

lion  13.5

rat  12.6

cat  12.1

mouse  12.1

jaguar  10.8

duck  10.8

dog  10.6

bottlenose dolphin  10.4

baboon  10.3

chimpanzee  9.7

guinea pig  9.4

human adolescent  9.0

human adult  8.0

pig  7.8

goat  5.8

cow  3.9

sheep  3.8

elephant  3.5

donkey  3.1

horse  2.9

giraffe  1.9

National Institutes of Health


Some animals have evolved truly remarkable adaptations to be able to sleep and survive in an otherwise inhospitable environment.  Consider advanced mammals like dolphins and whales for instance.  How do they sleep and not drown? 

These animals have developed the truly amazing capability of unihemispheric sleep, in which one side of the brain sleeps while the other side is awake.  Unihemispheric sleep allows dolphins and whales to sleep on one side of their brain while the other side stays alert.  This enables them to continue swimming and surfacing to breathe while part of their brain sleeps.

Birds that make long transoceanic or migratory flights, such as mallards, do the same thing – sleep on one side of their brain, then switch to the other.  This capability allows these animals to sleep while simultaneously tracking other group members and watching for predators.

Interestingly, some research suggests humans may also have a vestige of this capability, which might help explain the so-called “first night effect”.  This refers to the tendency some of us have to sleep poorly the first night we are away from our normal bedroom environment.  The study showed during the first night’s sleep away one hemisphere of the brain may have some limited response to external noises, while the other side does not.  The research also showed on subsequent nights the unihemispheric response was reduced.

Animals dream, too.  A considerable body of evidence shows both mammals and birds spend at least a portion of their sleep cycle in dream activity.

Many animals like dogs and cats are polyphasic sleepers, meaning they catch several naps throughout a 24-hour cycle, instead of having one unbroken phase of rest.  Humans are typically monophasic sleepers, meaning our bodies and minds have over time adapted to sleep normally over one sustained time period.