For most animals, sleep is an important process that serves several functions. In the case of non-human animals, sleep can be defined as a state where the animal exhibits altered consciousness, homeostatic regulation, or a reduction in the responses to changes in external stimuli. Going by the definition, sleep has been observed for several animals including mammals, some fish, some insects, amphibians, reptiles, and birds. An internal bodily clock known as the circadian clock determines the time that an animal goes to sleep or knows when it is time to sleep. This clock is synchronized to the movement of the sun and triggers sleep for both diurnal and nocturnal creatures.
Giraffes usually sleep, although not for long periods. In captivity, giraffes have been known to sleep for between four and five hours, which is mostly at night. During sleep, the animal may lie down or stay standing. Standing sleep is particularly common among older giraffes. The animal also has irregular sleep phases where the giraffe can enter into deep sleep. This form of sleep is usually characterized by the animal bending its neck and bringing its head to a rest on the thigh or the hip. Experts sometimes describe this position as paradoxical sleep, that is, a state where the animal is resting but there is rapid eye movement as well as reduced muscle activity.
Generally, dolphins go to sleep with only one side of their brain so that they use the other side of their brain for swimming and to watch out for predators. For some dolphins, they do not go to sleep in the first month of their lives. Instead, they rest by pressing their bodies against their mothers so that they do not drown. Therefore, it falls to the mothers to ensure that they do not go to sleep, which is why adults are able to go for at least a month without sleeping. In captivity, dolphins can enter a state of complete sleep while a tail reflex action keeps their blowhole above water.
Bullfrogs, which are scientifically known as Lithobates catesbeianus, do not go into a state that fits the definition of sleep. However, they do go into states of rest throughout the day. A study conducted in 1967 concluded that they do not go to sleep. In the study, the scientists delivered shocks to the frogs during the day and during the night when they appeared to be sleeping. The researchers expected that the response to stimuli should be slower during sleep. However, the responses were similar during the day and the night.
Elephants usually go to sleep at night and do so by lying down. In a day, the animal will go to rest for a period of between two and four hours. One of the reasons why they spend so little time sleeping is that they are vegetarians. Vegetation does not provide as much energy as other forms of food. Consequently, the animal has to spend around 18 hours a day feeding on around 600 pounds of vegetation daily in order to sustain its substantial size. Sometimes, they can sleep while standing against a mound or tree.
6. Alpine Swifts
The Alpine swift is a bird that lives in habitats ranging from southern Europe all the way to the Himalayas. These birds spend large amounts of their time migrating all the way to the southern regions of Africa. The birds may spend up to 200 days in flight during their migration. Just like some marine animals, the Alpine swift has the ability to sleep with one side of their brain while the other side focuses on flight and detecting predators.
Deer do not sleep that much during the day or the night. In fact, the night is a dangerous time for the animal to sleep due to the presence of several predators that hunt under the cover of darkness. However, deer manage to sleep for a few hours, which is usually between three and four hours. Even during these sleep periods, they have to be careful so that they do not fall prey to predators. The primary concern of the deer is survival, which forces them to sleep efficiently.
Ostriches sleep in a manner that resembles that of the platypus. The bird usually sleeps while standing upright with both eyes open. This state allows the bird to rest its brain and body while at the same time staying alert for any predators and invaders. Sometimes, the bird goes into a deeper state of rest, which is characterized by putting the head down for a period of around 15 minutes. These two types of rest add up to between six and seven hours sleep daily.
Horses have the ability to sleep for only two hours in a day. Just like giraffes, they are capable of sleeping for short bursts that last for around 15 minutes. Similarly, they can also sleep while standing or lying down. The young ones tend to sleep more than adults do. The animals sleep best in groups for additional protection, which is why a lone horse cannot go to sleep since its primary instinct is survival. Domesticated ones sleep for close to three hours in a day.
Some seals go to sleep with one side of their brain while others go to sleep completely. Others can go to sleep while completely submerged and come to the water surface to get a few breaths of air before going back down. Other species have been known to sleep while holding their nostril above the surface of the water during deep sleep. While on land, it is impossible for them to enter a state of deep sleep.
Due to their massive sizes, not all whales can afford to sleep for long as they can easily drown. Sperm whales sleep bobbing at the surface of the water in an upright position for about 5 to 10 minute periods. In fact, scientists suspect that they require the least sleep among mammals accounting for less than 10% of their daily activities.