Sleep in animals

Sleep is a biological necessity for almost all animals. It is a vital state that helps bodies and minds feel rested. Biologists define sleep in two primary ways. The physiological definition focuses on internal changes. This includes reversible unconsciousness, specific brainwave patterns, and sporadic eye movements. It also involves a loss of muscle tone. The behavioral definition looks at external actions. This includes minimal movement, a typical posture, and staying in a sheltered site. A key feature is sleep homeostasis. This means the longer an animal stays awake, the more intense and long its next sleep will be.

Most animals regulate their sleep using an internal circadian clock. This clock is driven by circadian genes. These genes undergo transcription that oscillates with time. This internal timing creates different biological sleep schedules. Diurnal species have waking periods that overlap with the day. Nocturnal species are active during the night. Crepuscular species are active during twilight, which is dawn or dusk. These rhythms likely evolved as a form of niche partitioning. This allowed different species to minimize competition by using different parts of the day.

Invertebrates provide evidence that sleep is an ancient evolutionary trait. Some of the most primitive organisms show sleep-like states. This includes the jellyfish *Cassiopea* and the freshwater polyp *Hydra*. Because jellyfish lack a complex central nervous system, they prove sleep does not require a brain. The nematode *Caenorhabditis elegans* also requires sleep. It enters a lethargus phase before each moult. This suggests sleep may be linked to developmental processes. In insects, sleep is also vital. For a long time, scientists did not believe insects slept. They changed their minds after studying the fruit fly, *Drosophila melanogaster*. They found that gene expression related to lethargy was similar to mammals.

Insects show many different sleep characteristics. While they do not seem to exhibit REM sleep, they do have circadian rhythms. Fruit flies show cognitive disabilities when deprived of sleep. In tests, sleep-deprived flies take longer to learn tasks and forget them more quickly. Other arthropods also show sleep signs. Cockroaches exhibit a rest period with folded antennae and decreased sensory sensitivity. Crayfish show changes in their EEG patterns during sleep. Some insects, like honeybees, even use sleep to store long-term memories. Jumping spiders have even shown bouts of retinal movements that look like REM sleep.

Marine animals have developed unique ways to stay safe while resting. Some species use unihemispheric sleep. In this state, only one cerebral hemisphere sleeps at a time. The other half stays active to maintain motion or homeostasis. This is seen in some porpoises and certain sharks. For example, great whites and hammerheads must move to move oxygenated water over their gills. It is suspected that some fish use this method to sleep while swimming. The octopus also has a complex two-stage sleep. First, it enters a "quiet sleep" stage for about 60 minutes. This involves closing its eyes and a flat body posture. Then, it enters an "active sleep" stage for about one minute. This stage features increased breathing and more eye movements.

Vertebrates show deep connections through their sleep patterns. Birds and mammals are the only animals known to exhibit both REM and NREM sleep. This suggests a common ancestor. Reptiles also show sleep-like states. Their daytime activity alternates between basking and active behavior. Scientists propose that REM sleep may have evolved from short bouts of motor activity in reptiles. Meanwhile, slow-wave sleep may have evolved from their basking states. In the Australian dragon, *Pogona vitticeps*, researchers found both REM- and NREM-like stages. Amphibians also have periods of inactivity, though they remain very vigilant to threats.

For many complex animals, the lack of sleep is a matter of survival. In rats, sleep deprivation is extremely dangerous. If rats do not get sleep, they can die within a few weeks. They often experience a decrease in appetite, leading to weight loss. This necessity is seen across many taxa. While some very simple animals with no brain may not follow these rules, most complex life requires rest. Sleep is a nearly universally conserved trait. This means it has been passed down through almost all animal lineages throughout history.