Sleep remains a mysterious thing on so many levels, but one thing is clear:  virtually every animal requires some form of sleep, or at least its equivalent.  Even the most simple creatures, like planaria, demonstrate regular periods of behavioral rest.

There are 4 generally accepted criteria for sleep universal in the animal world, and these obviously mirror how we understand sleep for humans as well:

1.  Little or no movement

2.  Stereotypic posture (most commonly lying down)

3.  Reduced response to stimulation

4.  Reversibility (permament sleep would be a problem)

Certain animal species can sleep in very unique ways, however.

Dolphins, for example, have unihemispheric “deep” sleep, in which one-half of the brain demonstrates sleep (as measured by electroencephalographic, or EEG, waveforms) while the other half demonstrates EEG evidence of wakefulness.  Such functional hemispheric disconnections may persist for minutes or hours at a time.  I find this endlessly fascinating.  Essentially, dolphins can truly be half-awake, half-asleep, perhaps to be able to swim and/or remain aware of surroundings or predators while at rest.

Various herbivores, like cows and sheep, can ruminate (chew and rechew cud) during sleep.

Many different animal species hibernate in order to reduce energy requirements in harsh environments.  These animals would include mammals (bears, bats, and marmots), certain birds, snakes, turtles, frogs, and snails.  How such animals arouse regularly from hibernation is among the many mysteries of animal physiology.

Some animals sleep in trees, both predators (like cheetahs) . . .

. . . and prey (such as baboons).

Adult giraffes generally don’t need much sleep.  It’s estimated that they get 1/2 – 1 hour of sleep per night, usually in very brief (5-minute) naps.  They may remain standing while sleeping.

Enjoy your weekend, everybody!  Cheers!