A species of fish that requires very little rest has shed light on the evolution of sleeping habits.

Neuroscientists from Florida Atlantic University are studying the Mexican Pachón cavefish in an effort to provide insight into the evolutionary mechanisms regulating sleep loss and the relationship between sensory processing and sleep. The fish have no eyes and live in deep, dark caves, with little food, oxygen or light. Due to this environment, they have evolved to suppress sleep.

The scientists are using this fish as a model to understand how human brains could evolve to require very little sleep.

To study the fish, the scientists recorded the cavefish under infrared light in individual tanks and used automated video tracking software that showed when the fish were inactive.

The new study suggests that an inability to block out the environment can interfere with sleep. It also provides a model for understanding how the brain’s sensory systems modulate sleep and gives scientists a better understanding into the evolution of the significant differences in sleep duration observed throughout the animal kingdom.

“Animals have dramatic differences in sleep with some sleeping as much as 20 hours and others as little as two hours and no one knows why these dramatic differences in sleep exist,” Alex Keene, Ph.D., a corresponding author of the study and associate professor in the Department of Biological Sciences in FAU’s Charles E. Schmidt College of Science, said in a statement.  “Our study suggests that differences in sensory systems may contribute to this sleep variability.

“It is possible that evolution drives sensory changes and changes in sleep are a secondary consequence or that evolution selects for changes in sensory processing in order to change sleep,” he added.

Keene was able to relate the study results to humans.

“Humans block out sensory cues when we enter a sleep-like state,” Keene said. “For example, we close our eyes and there are mechanisms in the brain to reduce auditory input. This is one of the reasons why sensory stimuli like someone entering a room is less likely to get your attention if you are asleep. Our thinking was that cavefish have to some degree lost this ability and this drives sleep loss.”    

The cave environment differs dramatically from the rivers inhabited by surface fish, which enables the cavefish to evolve in foraging and feeding behavior, raising the possibility that differences in nutrient availability can contribute to the evolution of sleep loss in cave populations.

Multiple cave populations have substantial reductions in sleep duration and enhanced sensory systems, which suggests that sleep loss is evolutionary and functionally associated with sensory and metabolic changes.

The research team showed that the evolution of enhanced sensory capabilities contributes to sleep loss in cavefish and that sleep in cavefish may be regulated by seasonal changes in food availability.

There are more than 29 different populations of cavefish, which largely evolve independently. This allowed researchers to determine whether evolution occurs through the same or different mechanisms.

For this study, the researchers studied the Pachon cavefish, which has appeared to lose sleep due to increased sensory input, while observing that other populations did not have this pattern.

“We were surprised to find that there are multiple independent mechanisms regulating sleep loss in different cave populations and this can be a significant strength moving forward,” James Jaggard, the first author and a graduate student at FAU, said in a statement. “This means that there are many different ways to lose sleep or evolve a brain that sleeps less and we are going to search to identify these mechanisms.”

The research team selected the cavefish to study because they are a powerful system for examining trait evolution. In previous studies they observed the evolutionary convergence on sleep loss in the fish.

However, the neural mechanisms underlying this behavioral shift remained unexplained, which led to the scientists examining whether an increase in sensory input from the neural conditions contributes to sleep loss.

 The researchers recently generated transgenic fish lines and will be able to image brain activity and genetically map anatomical differences between the Mexican cavefish populations.

The study was published in Journal of Experimental Biology.