Why sleep is nearly universal despite its costs
Sleep makes animals vulnerable: they lie down, close their eyes, lose consciousness, and cannot respond to predators. Yet sleep has persisted in virtually every animal species carefully studied, from insects to mammals [5]. This universality suggests that sleep evolved very early and that the benefits outweigh the risks. One hypothesis is that sleep optimizes neural plasticity—the brain's ability to reorganize and form memories—which may be essential for learning and survival [1].
Another idea is that sleep helps manage metabolic efficiency and reduce oxidative damage. According to this view, sleep may have evolved as a way to conserve energy and repair cellular damage, especially during times when being active would be costly or dangerous [3]. If sleep serves such basic maintenance functions, completely eliminating it would require evolving alternative solutions that are equally effective—a tall order.
Some animals can go without sleep for long stretches
Despite sleep's universality, a growing number of studies show that some species can forgo sleep for extended periods. For example, certain birds and marine mammals can sleep with only half their brain at a time, allowing them to stay alert for predators or to keep swimming [4]. This suggests that sleep can be flexible and that its need may be reduced under certain ecological pressures.
Even more striking, laboratory experiments with fruit flies (Drosophila melanogaster) have found that chronic sleep restriction does not necessarily shorten lifespan or cause obvious harm, challenging the idea that sleep is a vital necessity for survival [2]. These flies appear to cope with limited sleep, hinting that sleep's essential functions might be bypassed or compensated for under some conditions. However, the researchers note that sleep likely still serves important roles, and the flies may simply be able to tolerate sleep loss better than previously thought.
Why evolution hasn't produced a sleepless animal
If some animals can go without sleep for long periods, why hasn't any species evolved to eliminate sleep entirely? One reason is that sleep may be tightly linked to fundamental cellular processes that cannot be performed while awake. For instance, the brain may need periods of reduced sensory input to consolidate memories and clear metabolic waste [1][3]. Evolving a completely sleepless nervous system would require redesigning these processes, which may be developmentally or physiologically impractical.
Additionally, sleep may be a default state that is easier to maintain than to abolish. The genetic and neural circuits that regulate sleep are deeply conserved across species [4]. Mutations that eliminate sleep might disrupt other essential functions, making such animals unlikely to survive. Even species that forgo sleep for long periods still show signs of sleep-like rest and likely retain the underlying need, just suppressed or reorganized [4]. Thus, while sleep can be reduced or timed differently, completely removing it may be evolutionarily inaccessible.
Sources used in this answer
The need for a simple animal model to understand sleep
Sleep is conserved across the animal kingdom and may optimize neural plasticity, but the paper does not address adaptations that would enable animals to function without sleep.
Most sleep does not serve a vital function: Evidence from Drosophila melanogaster
Chronic sleep restriction in fruit flies does not necessarily reduce lifespan or cause obvious harm, suggesting sleep may not be essential for survival under some conditions.
Sleep Optimizes Time in the Effective Environment
Sleep may be linked to metabolic efficiency and oxidative damage reduction, and adaptations such as slow metabolism can reduce sleep needs.
Evolution of Sleep and Adaptive Sleeplessness
Some species can forgo sleep for extended periods, challenging notions about sleep's adaptive value, but they likely retain underlying sleep-like processes.
Why we sleep: The new science of sleep and dreams
Every animal species carefully studied to date sleeps, suggesting sleep evolved early and is maintained by strong selective pressure.