Why is sleep so important? It seems to be a universal property.

Answer

Indeed, sleep occurs and is important in just about all animals, from jellyfish to humans. One of the oldest hypotheses states that an animal that experiences periods of inactivity will be less likely to fall prey to predators. Especially when it is inactive during the period when it is most vulnerable. When an animal is active, it moves and increases its chances of getting close to a predator. A bat, for example, is most vulnerable to birds of prey during the day. It then sleeps, hidden somewhere, where it will not encounter birds of prey. During the day a bat does not catch insects because they are mainly active at night. In addition, it consumes energy, making a period of inactivity energy-saving – an added survival advantage, especially if food supplies are limited.

Other hypotheses state that sleep is important for the brain to make new connections between brain cells and thereby improve memory (young children and animals also sleep more during the period when the brain is at its strongest development). Brain and other tissues, especially during periods of inactivity (sleep), recover from any damage done during the active period.

Some of these hypotheses are supported by experimental evidence. Since sleep occurs in all animals, experimental evolution or natural selection has been studied in small animal models. Such animals, eg fruit flies, have a short life cycle, which means that many successive generations can be studied in experiments. Fruit flies in which sleep deprivation was induced, or whose variants had been selected that sleep less long, or which had mutants with short sleep durations, were found to have impaired memory, a shorter lifespan and less tolerant to stress than normal animals. In other words, sleep deprivation has a negative impact on “fitness”. Natural selection will therefore select these animals away in favor of animals that sleep more. It was also experimentally shown that fruit flies that sleep longer are more resistant to starvation. So, in periods of little food supply, “long sleepers” have an advantage.

So there is indeed an important evolutionary pressure on sleep in animals. In the future, it is hoped that the genes that play an important role in this regard can be identified with the help of the above-mentioned animal models. It can then be determined whether these genes also play a role in humans.