Are there any nonsleeping animals, and, if so, which is the largest? From an evolutionary point of v

Xin Jin
PhD Student
The Rockefeller University

Lech, AK, Poland

Are there any nonsleeping animals, and, if so, which is the largest? From an evolutionary point of view, it seems that nonsleeping animals would gain many advantages over their ordinary counterparts—mainly by having a greater amount of time to accumulate resources. Yet, as far as I know, everything sleeps, fish included. Why? Does living matter inherently need rest, is sleep somehow connected with the day-night cycle, or maybe energy expenditure would be too great in a nonsleeping animal?

Xin Jin
HHMI predoctoral fellow,
PhD Student,
Laboratory of Neural Circuits and Behavior,
The Rockefeller University
This question is easy to ask but difficult to answer. First, we need to address the definition of sleep. Sleep is usually defined as a naturally occurring state with reduced consciousness, suspended physical activity, or both. It is easy to observe sleep of human beings as a physical resting state; however, in some other species, sleep can take more subtle forms. For instance, dolphins seem not to sleep, but in fact they rest with one brain hemisphere at a time. Half of a dolphin’s brain is in sleep mode while the other half remains functional and responsive to the external world. Something like sleep is present in all vertebrates and even invertebrates, such as earthworms, which have a sleeplike behavior state in which they have low foraging activity and mainly dwell in one place.

The circadian rhythm of sleep is evolutionarily conserved and represents an adaptation of a particular organism to its environment. Conventionally, the sleep cycle is synchronized with the day-night cycle as a result of the Earth’s rotation. However, it can also depend on additional input such as an animal’s metabolic state (fasting-fed). Sleep seems to be conserved across the animal kingdom, but the form of sleep and sleep regulation vary in different species. We are still actively learning more and more about sleep and trying to understand what sleep really means to animals.

What is the benefit of sleep, and why do different animals have different needs for sleep? For a long time, sleep was thought to be responsible for energy conservation because sleeping animals have reduced metabolism and lower body temperature. This period of inactivity provides animals a resting time to conserve energy when foraging might be less efficient, for example, at night for a visually guided animal. From this perspective, sleep is a byproduct of evolution and the alternating day-night cycle because animals that sleep and conserve energy at night will have advantages over those who don't.

Is energy conservation the only function of sleep? During sleep, animals reduce energy rates by only 5–10%, which seems very low if energy expenditure is the sole gain for spending up to 50% of their lifetime sleeping. Moreover, during sleep, the brain wave pattern is fairly active and distinct, implying that sleep may be a complex state instead of a passive resting period for the brain.

Long-term sleep deprivation in humans leads to memory loss, delusions, and paranoia, and sustained sleep deprivation is lethal to animals. Emerging evidence suggests that animals use sleep to actively fix or restore certain biological machinery to maintain optimal performance. Immune functions are enhanced during sleeping states, and as a result, sleep deprivation severely affects overall health of rats and even flies. In human studies, it is reported that people with less than 7 hours of sleep on average were almost 3 times as likely to catch a cold than those with 8 hours of sleep or more. In addition, shift work promotes the risk of many pathologies, including coronary heart disease and diabetes, and sleep misalignment has been suggested to precede the onset of cancers.

Besides physiology, sleep may regulate cognitive function in the brain, such as memory processing and arousal. Slow-wave sleep is important in transforming short-term into long-term memories. Many mental disorders are associated with sleep disruption, including schizophrenia and depression. In modern genetic laboratories, scientists dissect the relationship between sleep and health by using model organisms and molecular biology. In recent decades, a panel of circadian clock genes that regulate the sleep-wake cycle has been discovered. Intriguingly, disruptions of these genes’ functions in various animal models have a broad effect on neural functions, including impairing cognitive functions. Understanding sleep disruption and the mechanisms of sleep can also help us to understand brain disorders and the higher brain functions of humans. So, are there any animals that need no sleep? As far as we know, all animals need some form of rest, indicating that sleep fulfills some basic physiological need. Sleep is one of the biggest riddles in biology. More and more evidence suggests that sleep rhythm disruptions are concurrent with immunological and cognitive disorders, implying a mechanistic overlap between the basic need for sleep and normal human physiological and neurological functions.

Acknowledgment

Thanks to Dr. Mike Young’s lab members for helpful input and discussions and to Dr. Cori Bargmann for revision and proofreading.