The deep and enigmatic evolution of sleep: How learning about animal sleep will help us improve our lives?
We sleep a third of our lives. Sleep is a crucial brain activity that makes cognition, memory, and learning possible. But we are not the only ones. All animals, from elephants to butterflies, sleep. A recent study by researchers from the Max Planck Institute for Biological Intelligence (Seewiesen, Germany) indicated numerous remarkable similarities among all living animal species, vertebrates and invertebrates alike, but also many striking differences.
The diversity in how members of the Animal Kingdom sleep serves to investigate the evolution of its states, but, more importantly, it contributes to our understanding of the deep, mysterious functions of sleep. Explaining the similarities and differences across the tree of life is essential to better experiencing this universal phenomenon and improving our sleeping habits.
Sleeping is a crucial brain activity and key to our survival
We spend about one-third of our day sleeping. A third of our lives are devoted to this crucial activity, yet sleep remains an enigma for scientists. Researchers have established theories about its functions, but its mechanisms remain under the veil of mystery. From a biological perspective, sleep is allegedly maladaptive since it is a state of reduced environmental awareness. Nevertheless, neuroscientific research undoubtedly indicates that sleep is fundamental to enhancing brain function, neuronal connectivity, and toxin disposal.
Decades of research have unanimously characterized the crucial role of sleeping in the brain. Sleep is fundamental to:
§ Transfer memories from short-term storage in the hippocampus to long-term storage in the cortex.
§ Produce synaptic downscaling, which resets connectivity levels, allowing information acquisition and energy conservation.
§ Allow neurons to rest and recover due to slow-wave off-states.
§ Remove metabolic waste products that originated during wakefulness via the glymphatic system in the brain.
§ Establish and maintain the map of the body in the sensorimotor cortex, a brain area that receives sensory input from the body and sends commands to move.
Despite the known benefits for our well-being, sleep is one of the most affected activities by daily stressors. Adult humans sleep less than seven hours a day (Sleep Foundation, 2022). In the United States alone, between 50 to 70 million adults are affected by a sleep disorder (American Sleep Association, 2021). These staggering numbers equal the sum of California and Texas populations, the two most inhabited states in the US.
According to Aging research, the three most important factors for life longevity are adequate physical activity, a balanced diet, and, unsurprisingly, high-quality sleep. Getting enough and good sleep lowers the risk of diabetes, obesity, heart disease, and mood disorders. It becomes critical, then, that sleep must be protected at all costs. And, as with every biological phenomenon, there is an evolutionary reason behind its pivotal importance.
The omnipresent survival key of all animals has a deep evolutionary history
REM sleep is the behaviorally defined state characterized by wake-like electroencephalogram (EEG) activity (frequencies ranging from 4 to 32 Hz), rapid eye movements, loss of muscle tone, and twitches. By contrast, Non-REM sleep is defined by EEG slow waves (0.5 to 4 Hz), spindles, the absence of rapid eye movements and bodily movements, and some muscle tone.
Outstandingly, the discovery of a single sleep mode in jellyfish indicates that sleep evolved much earlier than previously suggested. According to the authors, REM and Non-REM stages are found in both mammals and birds, suggesting that sleep diversified into these two states before the Synapsida (mammals) and Sauropsida (birds) divergence around 320 million years ago, rather than evolving independently in these two amniote lineages. Interestingly, different modes of sleep according to behavior (quiet vs. active) and brain activity (REM vs. Non-REM) have been reported across the entire animal phylogenetic tree, from the zebrafish to the freshwater polyp.
Understanding the evolution of sleep will help us live better and longer
Sleep is expressed today as the byproduct of millions of years of adaptations. The components of sleep were gained, modified, repurposed, and lost as a response to a myriad of selective pressures. As the deep-rooted evolutionary protagonist of our survival as species, sleep is at the heart of high-quality life.
The promising future of science and medicine is pointing to the developments that allow balanced, longer lives. Consequently, evolutionary studies, such as the one portrayed here, contribute to a more comprehensive understanding of the functions and benefits of sleep and, therefore, to enjoy a better life.
References
Brocklehurst, N., Ford, D. P., & Benson, R. B. (2022). Early origins of divergent patterns of morphological evolution on the mammal and reptile stem-lineages. Systematic Biology, 71(5), 1195–1209.
Rattenborg, N. C., & Ungurean, G. (2022). The evolution and diversification of sleep. Trends in Ecology & Evolution.
About the author:
I am a Ph.D. candidate in Anthropological Sciences at Stony Brook University. I study the evolution of higher cognition, learning, habit formation, and decision-making in mammals by the application of phylogenetic comparative methods. As a young science communicator, my mission is to provide high-quality and vetted information in a sophisticated, straightforward, and appealing format.
