Some cave-dwelling fish may be capable of living without sleep.

Sleep is an ancient mechanism of the animal kingdom that emerged at an early stage of evolution. It is crucial for the health of not only humans but also other animals. Sleep allows individuals to restore their energy before a new day and consolidate their experiences in memory. The amount of time required for adequate rest varies among biological species. Some species can sleep for six months (such as bears that hibernate for half a year), while others may only sleep for a couple of hours each day (like elephant seals that sleep for two hours a day).

In the wild, there are animals that can stay awake for months, particularly during extended feeding periods or mating seasons. However, there are also species that do not require sleep at all, managing to live practically their entire lives without it.

For instance, one species of cavefish that inhabits the underground waters of Mexico and the United States exhibits this behavior. This refers to several populations of the Mexican tetra (Astyanax mexicanus). Despite the lack of sleep, these fish appear healthy, and their lifespan does not diminish. This discovery was made by a team of American and Mexican ichthyologists, as detailed in their article published on the electronic archive website for scientific articles and preprints in biology, bioRxiv.org.

Scientists had hypothesized for several decades that Mexican tetras could go without sleep. Researchers reached this conclusion while studying Astyanax mexicanus in laboratory settings. In their new study, a team led by Alex Keene from Texas A&M University has demonstrated for the first time that these fish do not sleep even in their natural habitats.

Most Astyanax mexicanus live in rivers outside of caves, possessing normal vision and gray or white-pink coloration. In contrast, 34 populations residing in caves have partially or completely lost their eyesight and coloration.

Keene and his colleagues studied fish from four populations of Astyanax mexicanus living in surface rivers, six populations inhabiting cave waters, and five populations of a so-called hybrid form (which retained some pigmentation and partial vision).

The specialists conducted visual observations and found that surface-dwelling fish sleep between three to six hours a day, depending on their activity level, while cave-dwelling and hybrid fish do not sleep at all.

According to Keene, the next step for his team is to record brain activity and investigate the physiological differences between fish living in surface waters and those in caves. Future research may reveal that cave populations of Astyanax mexicanus actually can sleep, but they exhibit unilateral sleep — a slow-wave sleep that occurs only in one hemisphere of the brain (similar to whales, dolphins, and certain bird species). During this type of sleep, the other hemisphere of the brain remains awake.

If this is the case, the cave-dwelling Mexican tetras might be able to swim while sleeping, but it remains unclear why they would engage in such behavior if their surface-dwelling counterparts do not.

Keene's team has several explanations. First, cave fish may have developed this trait because they need to spend more time searching for food. Secondly, they have developed such sensitive hearing that even the slightest noise prevents them from sleeping. The cave populations of Astyanax mexicanus possess lateral lines — a sensitive organ that detects movement and vibrations in the surrounding water. Earlier, Keene and his colleagues discovered that these lateral lines are extremely sensitive and may be linked to sleep suppression.

If it turns out that some populations of Astyanax mexicanus are indeed always awake, it would provide compelling evidence that this form of "insomnia" is an important and necessary element of adaptation to cave life.