For most animals, sleep is an important process that serves several functions, on top of the basic fact that we all love a good night's rest!
For non-human animals, sleep can be explained as a state of altered consciousness, homeostatic regulation, and a reduction in response to changes in external stimuli. This has been observed in several animals, like many mammals, amphibians, reptiles, birds and some fish and insects. The internal circadian clock, which is a 24-hour cycle aligned with the patterns of the sun allows animals to know when it is time to sleep.
But, some do not sleep or sleep very little or at the very least sleep in ways that are barely recognizable to us as sleep. Here are some animals who survive just fine without their eight hours.
In captivity, giraffes sleep just for four or five hours, mostly at night, but in the wild giraffes will only sleep for short bursts at a time. You might see them sleeping while laying down or, oddly, standing up, which is especially common among older giraffes. Giraffes can also enter into an irregular deep sleep phase, where they bend their neck and rest their head on the thigh or hip. Experts call this position paradoxical sleep: a state where they are resting but with rapid eye movement and reduced muscle activity.
Dolphins accomplish something called unihemispheric sleep: only half their brain sleeps, while the other half ensures the dolphin is close to the surface to breathe and stays on guard for predators. They alternate between the two hemispheres to get sufficient sleep for their whole brain.
These mammals do not sleep in the first month of their lives (can you imagine!). Instead, they rest by pressing their bodies against their mothers so that they do not drown, and the mothers, in turn, have to stay awake while the little ones rest, so they become accustomed to going a month without sleeping in adulthood as well.
Amazingly, horses can get by with only two hours of sleep a day, though domestic horses go up to three to five hours, and younger horses need more sleep than adults. They sleep for very short bursts of time, about 15 minutes, and can also sleep standing up or lying down, in paradoxical sleep like giraffes. Horses best sleep in groups for additional protection. Since their primary instinct is survival, they cannot go to sleep on their own.
4. Alpine Swifts
The Alpine swift is a bird that migrates to habitats ranging from southern Europe all the way to the Himalayas. These birds may spend up to 200 days in flight during their migration to the southern regions of Africa. Just like some marine animals, the Alpine swift can sleep with one side of their brain while the other side focuses on flight and detecting predators.
Bullfrogs do not go into a sleep state, but they do go into states of rest throughout the day. In 1967, scientists delivered shocks to the frogs during the day and night when they appeared to be sleeping. Although the researchers expected the response to stimuli should be slower during sleep, the responses were similar during both the day and the night. The only time they sleep properly is when hibernating.
Not all whales are able to sleep for long periods as they can easily drown due to their massive size! For example, sperm whales sleep in a vertical position while bobbing at the water's surface for only five to fifteen minute periods. In fact, with sleep accounting for less than 10% of their daily activities, scientists suspect that whales require the least amount of sleep among mammals.
Ostriches are funny creatures. Believe it or not, they actually sleep standing upright with both eyes open (much like the platypus) for about six hours at a time. Doing this lets these birds rest both brain and body while actually remaining alert to predators and invaders. And when they are really tired, ostriches can get into a deeper state of rest where they put their head down for about fifteen minutes.