Have you ever wondered why every creature from the smallest insect to the grandest whale succumbs to the vulnerable state of sleep? Despite the apparent danger and the evolutionary disadvantage of being momentarily oblivious to the world, sleep is a universal phenomenon among animals. This begs the question: why has such a seemingly risky behavior persisted throughout evolution? A new unified theory offers an astonishing explanation, proposing that sleep, in its essence, stems from an ancient symbiotic relationship. It goes back to when the earliest animals formed partnerships with microscopic organisms, leading to the development of mitochondria, the powerhouses of our cells. This theory not only demystifies the nature of sleep but also sheds light on the roles of quiet (NREM) and active (REM) sleep, pushing the boundaries of our understanding of this ubiquitous aspect of life.

A groundbreaking study, scientists Dr. Graham Adams from Perth, Australia, and Dr. Philip O’Brien from Murdoch University have unveiled a unified theory of sleep, shedding light on the endosymbiotic origins of this vital biological process. Published in the Neurobiology of Sleep and Circadian Rhythms, their research posits that the inception of sleep is intertwined with the evolutionary event that led to the development of mitochondria, offering profound insights into how these cellular powerhouses influence sleep patterns.

Dr. Adams elucidates the foundation of their research, “The origin of sleep is believed to be closely tied to the endosymbiotic event that led to the formation of mitochondria.” This perspective suggests that sleep is a complex outcome of our cells’ ancient relationships.

Further exploring host-parasite interactions, Dr. Adams draws parallels, stating, “These parasitic interactions provide a model for understanding how mitochondria could influence sleep behaviors in their hosts.” Such comparisons help illustrate the significant impact of microscopic organisms on sleep regulation.

Highlighting the biochemical role of mitochondria, Dr. Adams shares, “Mitochondria are not only crucial for aerobic respiration but also for the production of neurotransmitters like GABA and DOPA, integral to our sleep/wake cycles.” This underscores the dual function of mitochondria in energy production and sleep regulation.

The significance of this research extends far beyond academic curiosity, delving into the intricacies of sleep’s evolutionary journey, highlighting the indelible mark left by our cellular ancestors on our daily lives. Through understanding the mitochondrial imperative for sleep, Dr. Adams and Dr. O’Brien pave the way for novel approaches to sleep disorders, potentially revolutionizing treatments and enhancing life quality. This unified theory not only contextualizes sleep within the vast tapestry of life on Earth but also invites us to reconsider our relationship with the microscopic worlds within us. By bridging gaps between disciplines, Dr. Adams and Dr. O’Brien’s work underscores the profound interconnectedness of life, from the cellular to the cerebral. Their insights into the endosymbiotic origins of sleep underscore the complex interplay between biology, environment, and evolution, offering a compelling narrative that reshapes our understanding of one of life’s fundamental processes.


Dr. Graham Adams and Dr. Philip O’Brien, “The Unified Theory of Sleep: Eukaryotes’ Endosymbiotic Relationship with Mitochondria and REM the Push-Back Response for Awakening,” Neurobiology of Sleep and Circadian Rhythms, 2023.

DOI: https://doi.org/10.1016/j.nbscr.2023.100100


Dr Graham Adams PhD

I’ve been fascinated by the strange behaviour of sleep for decades. It started when I was watching Claudia my Rottweiler, sleep then wake then sleep, about every 9 minutes. Intrigued, I filmed other dogs in their natural habitats and was amazed to find they wake up about 20 times a night. Also, when they sleep with other dogs they wake at different times.
Over the decades I studied more about sleep in dogs, humans and other animals. I was captivated when a paper came out saying that like humans, dogs needed to sleep in order to remember. However, as a Behaviour Specialist of dogs and cats, I thought the dogs were trained wrongly and the results may be wrong. So I conducted my own experiment using dog’s sense of smell instead of sight. The dogs did not need to sleep in order to remember. This led me to other studies where animals didn’t need to sleep to consolidate memory, such as neonate whales who don’t sleep for their first month.
I wondered whether sleep did not have a function. Maybe it was just an occurrence. However I realised that there must be something very powerful driving it for evolution to have selected it. So, it was necessary to look across 14 different biological disciplines to find the answers which led to “The Unified theory of sleep: Eukaryotes endosymbiotic relationship with mitochondria and REM the push-back response for awakening.”
I was an Honorary Research Fellow at The School of Veterinary Studies Murdoch University Western Australia. I continue with my passionate interest in how animals perceive their world and how they think. I have regularly appeared on Australian national TV and radio and I’m the author of numerous peer reviewed scientific publications in international journals. orcid.org/0000-0002-8443-6092

Philip O'Brien

Dr. Philip O’Brien, Ph.D

Sleep research was not something that excited me as a young researcher. I was more interested in molecular biology, and it was a thrilling time to be a molecular biologist.  Techniques for cloning and sequencing DNA were being refined rapidly and we were on the cusp of achieving the holy grail of sequencing the genomes of higher eukaryotic plants and animals.  My field of research was plant diseases, specifically fungal diseases of cereals, and oomycete diseases of trees and shrubs.  We could use the techniques of molecular biology to elucidate disease mechanisms, and then genetically engineer plants to be resistant to these diseases.  I had known Dr G Adams for many years but given that our research areas were very different we had never established a research collaboration.  That did not happen until we were both retired, and we got to tossing around ideas of sleep but coming at it from different angles.  This has proved to be an interesting, pleasant and fruitful collaboration.