They don’t snore, and they don’t dream – but jellyfish and sea anemones were the first to present one of sleep’s core functions hundreds of millions of years ago, among the earliest creatures with nervous systems.
A groundbreaking new study from the faculty of life sciences and the multidisciplinary brain research center at Bar-Ilan University has just been published in the prestigious journal Nature Communications under the title “DNA damage modulates sleep drive in basal cnidarians with divergent chronotypes.”
The study was led jointly by the laboratories of Prof. Lior Appelbaum and Prof. Oren Levy at BIU. In previous research, the Appelbaum Lab showed in zebrafish that neurons accumulate DNA damage during wakefulness and require sleep to recover, highlighting the need to reduce DNA damage as a fundamental driver of sleep.
“Sleep is an evolutionarily conserved process that is vital for animal survival. Sleep disturbances affect a large portion of the general public and represent a major health burden. Although sleep clearly improves brain performance, the function of sleep is unclear,” Applebaum noted in an interview with The Jerusalem Post.
The team wanted to know why sleep evolved. “We thought that it involved not only the whole brain in some creatures but even a single neuron – both of which need cellular maintenance – so we focused on the earliest creatures that have nervous systems,” he said.
“It’s important not just for improving learning and memory, but also for keeping our neurons healthy. The evolutionary drive to maintain neurons that we see in jellyfish – the team studied upside-down jellyfish (Cassiopea andromeda) and sea anemones (starlet sea anemone Nematostella vectensis) – is perhaps one of the reasons why sleep has evolved and is essential for humans today,” Appelbaum said.
Characterizing sleep patterns in ancient animal lineages
IN THE current study, Dr. Raphael Aguillon, Dr. Amir Harduf, and colleagues from the Appelbaum and Levy labs defined and characterized sleep patterns in two ancient animal lineages: diurnal, symbiotic jellyfish that sleep at night and shortly nap at mid-day, and “crepuscular” sea anemones that fall asleep before sunrise and sleep during the first half of the day.
DNA damage can arise from a variety of sources, including neuronal activity, oxidative stress, metabolism, and radiation. The cell repair systems fix these DNA breaks to avoid “genome insults” [mutations in genetic material from internal or external factors] with failures leading to aging, cancer, or disease. These can range from single DNA lesions to large chromosomal changes that affect cell function and overall health. Neurons may require sleep to facilitate DNA and cellular maintenance because they are unique non-dividing excitable cells.
“Our findings suggest that the capacity of sleep to reduce neuronal DNA damage is an ancestral trait already present in one of the simplest animals with nervous systems,” said Appelbaum, who is the principal investigator of the Molecular Neuroscience Lab at BIU’s Faculty of Life Sciences and Multidisciplinary Brain Research Center. “Sleep could have originally evolved to provide a consolidated period for maintenance of the neurons – a function so fundamental that it may have been preserved across the entire animal kingdom.” When the animals were kept awake and DNA damage increased, they slept longer afterward. This behavior, known as sleep rebound, made possible recovery as well as the reduction of DNA damage levels.
By tracing this mechanism back to these ancient animals, he continued, “we found that protecting neurons from DNA damage and cellular stress is a basic, ancient function of sleep that began long before complex brains evolved.
“We have characterized sleep, cloned sleep genes, visualized sleep circuits, and established zebrafish models for sleep disorders,” he continued.
The brain repairs itself round the clock, but apparently requires an offline period to reduce cellular stress that accumulated during wakefulness. If the sleep schedule changes regularly, sleeping sometimes at night and other times during the day, it may affect neuronal health. People who chronically sleep too little – including those suffering from obstructive sleep apnea in which they stop breathing many times per night – can increase their risk of neurological damage, including Parkinson’s and Alzheimer’s disease.
Every species sleep needs differ
EVERY SPECIES needs a different amount of sleep. “Koalas and dogs need more. Most birds go to sleep and rest their brains, but migrating birds including swifts, mallard ducks, and some sandpipers can sleep with half their brain – allowing one hemisphere to rest while the other stays alert with one eye open,” Applebaum explained. “This is essential for long migrations and continuous flight. Marine mammals such as dolphins and sea lions do this too because if their brains had to rest completely, they wouldn’t be able to breathe – they would drown.”
Levy’s lab has for years raised large numbers of jellyfish that originate in the Red Sea and sea anemones from the US. He and his team focus on studying coral reefs and reforming them, light pollution, biological clocks, sleep disorders, and symbiosis. Looking ahead, Levy said he wants to study sponges and other creatures that have no neurons and are even older than jellyfish and sea anemones to see if they sleep or not.
Awareness of the environment declines during sleep, leaving animals more exposed to predators and interrupting vital behaviors such as feeding and reproduction. The fact that pet dogs sometimes sleep on their backs with their legs in air shows trust in their owners, Levy stated. “The fact that they can sleep 14 hours a day or more shows that the need to reduce cellular stress and remove neuronal DNA damage is more important than being exposed to danger while they snooze. According to the study, the indispensable function of sleep appeared early in animal evolution and is so crucial that it outweighed its inherent dangers.”
The study also showed that increasing DNA damage, either through ultraviolet radiation or exposure to a chemical that damages the acid, triggered recovery sleep in both species. Conversely, promoting sleep with the hormone melatonin reduced DNA damage. These findings reveal a bidirectional relationship in which such damage increases sleep need, and sleep, in turn, facilitates damage reduction, suggesting that protecting neurons from daily cellular stress and DNA damage may have been the evolutionary driver of sleep.