Sleep (zoology)
Sleep in zoology refers to a state of reduced activity and awareness characterized by several key features: prolonged physical immobility, diminished sensitivity to environmental stimuli, specific sleeping sites and postures, and a circadian pattern of activity. Almost all animals exhibit some form of sleep, with behavior and physiology influencing how they sleep. The ecological context shapes sleep patterns, as animals often select secure environments or postures that enhance safety from predators while also considering temperature regulation.
Sleep can be broadly categorized into nonrapid eye movement (NREM) and rapid eye movement (REM) stages, with variations across species. Birds and mammals demonstrate alternating cycles of NREM and REM sleep, while other animals may experience simpler patterns of brain wave changes associated with sleep. Research indicates that larger animals tend to sleep longer and more deeply than smaller animals, and young animals consistently show increased total sleep time compared to adults.
The potential functions of sleep are diverse, encompassing learning reinforcement, visual-motor coordination, and thermoregulation. Overall, sleep serves crucial roles in maintaining physiological health across different animal species, aiding in processes such as cellular regeneration and immune support. Understanding sleep in animals provides insights into their behavioral ecology and evolutionary adaptations.
Sleep (zoology)
Behaviorally, sleep can be recognized by four basic features. It generally consists of 1) a prolonged period of physical immobility during which there is 2) reduced sensitivity to environmental stimuli, 3) typically occurs in specific sites and postures, and 4) in a twenty-four-hour (circadian) pattern. Using this broad definition of sleep, almost all animals can be said to sleep.
The Ecology of Sleep
Since animals are more vulnerable to danger when sleeping than when awake, most animals sleep in sites and postures that help to maximize their safety. For an insect or a small lizard, this might mean wedging into a crack in tree bark or burying themselves under leaf litter. For a snake, small bird, or mammal, it might mean sleeping in a nest, burrow, or tree hollow. Some animals can adopt a particular sleep posture that helps them blend into the background to avoid detection. Animals that cannot hide or camouflage themselves might try sleeping while semiprotected in the center of a family or larger group.
Animals can also modify their sleep sites and postures to help regulate their temperature. In cold temperatures, sleep sites and postures can be chosen to cover exposed skin on the face or feet; birds fluff their feathers, and mammals fluff their fur to trap air like a blanket; small animals huddle together to keep warm. In particularly hot temperatures, well-chosen sleeping sites may protect an animal from direct exposure to the sun, and specific postures can be adopted to facilitate heat loss.
Sleep periods also tend to be taken at times that are most safe. Diurnal species are those that are typically active in the day and do most of their sleeping at night; nocturnal species are those that are more active at night and do most of their sleeping during the day. In general, birds, reptiles, and shallow-water species tend to be active during the day, while mammals and deep-water species tend to be active at night, but there are many exceptions to this generalization. Whether a particular species is primarily diurnal or primarily nocturnal depends upon many aspects of its ecology and physiology, but most exhibit some kind of circadian pattern of rest and activity.
The Physiology of Sleep
Neurophysiologically, “sleep” can be distinguished from “rest” in vertebrate animals only by measuring changes in brain state. Fish, amphibians, reptiles, birds, and mammals all show changes in brain waves that accompany the progressive muscle relaxation that characterizes deeper and deeper states of sleep.
During their sleep periods, fish, amphibians, and reptiles slowly progress into more relaxed stages of sleep, then remain in their deepest state for a prolonged period, eventually returning slowly back to the waking state. Birds and mammals, on the other hand, show a pattern of alternating states of sleep within each sleep period. The first state is called nonrapid eye movement sleep (NREM). NREM sleep is characterized by relaxation of the muscles, slowed breathing and heart rate, and slow, synchronous waves in the brain's neocortex according to an electroencephalogram (EEG), a measure of brain activity. Alternating with periods of NREM sleep are periods of REM sleep. REM sleep is characterized by rapid eye movements, irregular heart rate and breathing, and fast, paradoxical wake-like waves in the EEG that look identical to brain activity while awake. Although the brain is very active during REM sleep, most muscles are deactivated, leading some people to refer to REM sleep as “paradoxical sleep.” In humans, dreaming typically occurs during REM sleep.
Some birds and marine mammals can sleep on one side of their brain and body while the other side remains awake. In marine mammals, it is thought that one-sided sleeping may enable an animal to keep swimming and stay near the surface to breathe. In birds, one-sided sleeping is thought to be a way for particularly vulnerable species to get some rest while remaining alert for predators.
For decades, scientists only observed REM and NREM sleep among mammals and birds. With twenty-first-century technology and scientific testing procedures, scientists identified similar REM, silencing behaviors, and slowed brain activity sleep cycles in flies, worms, fish, reptiles, and cephalopods.
Across-Species and Developmental Patterns of Sleep
Large animals have longer sleep periods than small animals. Large animals also sleep more deeply (are more relaxed and have fewer arousals) than small ones and, among birds and mammals, have a greater proportion of REM sleep. According to the vigilance model of sleep, this is because large animals are less vulnerable than small animals and so can afford to be less alert. Supporting this idea is the fact that for a given size animal, species that are predators typically sleep more and sleep more deeply than animals that are prey. Cougars, for example, sleep more and sleep more deeply than the deer they hunt, while falcons sleep more and sleep more deeply than pigeons and ducks.
Although large animals tend to have longer sleep periods than small animals, small animals generally have more total sleep time than large animals because they sleep more often. It is not known whether this pattern results because small animals need more sleep or because what sleep they do get is shallower and more disrupted.
Consistent with the fact that small animals sleep more than large animals is that in any particular species, young animals sleep more than adults. Not only does total daily sleep time drop as an animal ages, but so does the relative percentage of time spent in REM sleep. Human babies have more total sleep time and a greater percentage of REM sleep than adults and young adults have more sleep time and a greater percentage of REM than older adults. The same pattern seems to hold true for other species as well.
Possible Functions of Sleep
Besides the vigilance model, three other models attempt to explain across-species and across-age differences in sleep. One of these suggests that sleep is necessary for learning because, during sleep, synaptic connectivity is sharpened and reinforced. Since large animals generally live longer than small animals, they typically have a greater learning capacity. Likewise, for a given size animal, predatory species typically rely more on learning, while prey species rely more on instinct. (A prey animal who makes a mistake is dead, whereas a predatory animal who makes a mistake can always try again.) According to this model, larger animals and predatory animals not only can afford to sleep long and deeply, but they actually need more sleep to process and encode information. This model also accounts for the fact that young animals sleep more than older ones (they have more to learn) and that after accounting for body size and predator/prey status, mammals sleep more than birds.
A second model suggests that sleep is necessary for visual-motor coordination. This model was originally formulated to try to explain why birds and mammals have REMs during sleep, but fish, reptiles, and amphibians do not. Birds and mammals have a much more complex visual system than other vertebrates. This model also attempts to explain why young animals sleep more than older animals—their visual and motor systems are not yet fully developed—and why young animals of altricial species (those born or hatched relatively helpless) sleep more than young animals of precocial species (those born or hatched at an advanced stage of development).
A third model suggests that sleep functions as a mechanism for thermoregulation. Among warm-blooded species, small animals, having a greater surface-to-volume ratio, both lose heat and overheat more rapidly than large ones; thus, they would need to rest more frequently but for shorter periods. Young animals, according to this model, need to sleep more than older animals because their thermoregulatory abilities are not yet fully developed. Likewise, altricial animals sleep more than precocial animals because their thermoregulatory mechanisms are less well developed.
Many functions of sleep generally apply to all vertebrates. For example, NREM sleep restores nervous system function by promoting cellular regeneration, repairing neural connections, and clearing brain cells of harmful contents. NREM also helps regulate hormones, repair muscles, and support the immune system.
In vertebrates and invertebrates, sleep is regulated, at least partly, by the proteins called AP-2 transcription factors (the TFAP2 gene instructs the production of these proteins). Sleep also replenishes the brain glycogen levels that deplete during waking hours in vertebrates and many invertebrates. Glycogen contributes to energy, metabolism, and neurobehavioral functions like memory and learning.
Principal Terms
Circadian Rhythm: a physiological or behavioral cycle that occurs in a twenty-four-hour pattern
Diurnal: habitually active during the day
Electroencephalogram (EEG): a chart of brain wave activity as measured by electrodes glued to the surface of the skull
Nocturnal: habitually active during the night
Nonrapid Eye Movement (NREM) Sleep: sleep characterized by relaxed muscles and slow brain waves
Rapid Eye Movement (REM) Sleep: sleep characterized by fast brain waves, during which dreaming typically occurs
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