The amount of rest an animal needs varies drastically across the animal kingdom. Some species function perfectly with just a few hours of rest, while others spend nearly their entire lives in a state of slumber. This incredible variability reflects a multitude of factors, including diet, metabolism, and the pressures of survival in a hostile environment.
The Reigning Champion of Daily Sleep
The undisputed titleholder for the longest average daily sleep duration among mammals is the koala, which can spend between 18 and 22 hours per day resting. This extreme dedication to sleep is a direct consequence of its specialized, low-energy diet. Koalas feed almost exclusively on eucalyptus leaves, which are fibrous, low in nutritional value, and contain toxic chemical compounds.
The koala’s solution is to conserve as much energy as possible by remaining largely inactive and sleeping for nearly 90% of a 24-hour cycle. Other champions of daily rest, such as the little brown bat, also exhibit prolonged sleep, averaging around 19.9 hours daily to conserve the energy used during their high-metabolism, nocturnal insect hunts.
Differentiating True Sleep from Extended Dormancy
When discussing long periods of animal inactivity, it is important to distinguish between true sleep and controlled states of extended dormancy. True sleep is a predictable state of reduced consciousness following a circadian rhythm, necessary for cognitive function and memory consolidation. Dormancy, by contrast, involves a profound physiological shutdown to conserve energy during harsh environmental conditions.
The term hibernation describes a long-term, seasonal state of metabolic suppression that can last for weeks or months. During true hibernation, as seen in the Arctic ground squirrel, the animal’s body temperature plummets, sometimes dropping to near or even below freezing, and the heart rate and respiratory rate slow dramatically.
A related, but shorter-term state is torpor, which often lasts less than 24 hours. Daily torpor is a temporary drop in body temperature and metabolic rate, often used by small mammals and birds to survive a single cold night or a brief period of food scarcity. Hibernation, therefore, is better described as a prolonged, multiday bout of torpor, a strategy for survival rather than a daily restorative requirement.
Animals That Hardly Sleep At All
Species that manage to survive and thrive on little rest exist at the opposite end of the spectrum. Wild African elephants, for example, have been recorded sleeping for only about two hours per day, making them the shortest-sleeping mammals currently known. Due to their size and the constant need to forage for enough food, they cannot afford long periods of rest and often sleep in short, polyphasic bursts while standing.
Giraffes also sleep very little, often averaging just a few hours of sleep per day, which they primarily take in quick naps lasting only a few minutes each. This behavior is a survival adaptation; lying down makes these large animals extremely vulnerable to predators, as rising takes time and effort. Many marine mammals, such as dolphins and whales, have evolved unihemispheric slow-wave sleep. This mechanism allows one half of the brain to rest while the other remains awake to control breathing and maintain vigilance for danger.