Coral Sleep Study Reveals How Microbes Shape Night-time Rest

Sleep appears straightforward, until examined more closely. Most animals sleep, and there are good reasons for it.

During this quieter period, cells repair the day’s damage, mend tissues and reset the body’s internal “do we keep going or do we shut down?” controls.

Yet sleep also carries risks. An animal that is asleep is less able to flee, conceal itself or defend itself. Why, then, has nature retained sleep for so long?

Here is the unexpected part: researchers have identified sleep-like resting states throughout the animal kingdom, from mammals to tiny invertebrates.

One major question remained, however: what happens in animals that emerged so early in evolution that they have no neurons at all?

This is where corals enter the picture. A new study reveals something remarkably familiar: even an organism resembling living stone needs to stop for the night.

A quiet reef at sundown

The research examined a living coral reef near Curaçao in the Caribbean Sea. Scientists studied a brain coral species known as Pseudodiploria strigosa.

Like many corals, it does not live alone. Microscopic partners inhabit its cells; in this instance, the partner is an alga called Breviolum. The alga produces food through photosynthesis and shares it with the coral.

Rather than using a laboratory tank with regulated lighting and water conditions, the researchers observed the system in its natural reef environment.

That distinction matters because reefs contain countless changing elements. Light can alter rapidly, water chemistry varies and numerous organisms interact continuously.

The work was led by the Institute of Evolutionary Biology (IBE), a joint research centre of the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF).

Six-hour dives, day and night

To track the coral and its microscopic partners over a full daily cycle, the team dived every six hours for three days. Their work took place at a depth of roughly 5 metres.

On every dive, they gathered samples and analysed gene expression, checking which genes were active or inactive at different points of the day.

This revealed a distinct pattern: the coral itself enters a resting state at night, while the microbes living alongside it remain active.

“P. strigosa sleeps for a third of the day, like humans, and the day-night cycle regulates its biological clock, known as the circadian rhythm,” said Bradley Allen Weiler, first author of the study.

That finding is significant in itself. Corals have neither brains nor nervous systems, but they nevertheless follow a daily rhythm in which night-time is devoted to restoration and recovery.

Coral sleep while microbes remain active

Perhaps the most striking finding is how unequal the partnership becomes after dark. As night begins, the coral switches on repair mechanisms, including processes that mend DNA damage accumulated during the day.

In daylight, the algae inside the coral carry out photosynthesis. This supports food production, but photosynthesis also generates reactive oxygen species: chemically reactive molecules capable of damaging cells and DNA. The coral is therefore nourished, but also harmed.

“At night, the corals repair DNA damaged by their symbionts,” said Javier del Campo, an associate professor at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.

“The microbes stop photosynthesizing, but they maintain other cellular functions and do not need a long rest.”

The coral’s night-time rest is therefore more than a welcome pause. It appears to be a built-in safeguard that prevents a beneficial relationship from becoming toxic.

A partnership governed by limits

It is tempting to describe symbiosis as friendship, but nature is not driven by goodwill. The study highlights that reality.

Corals and their microbes both gain from the relationship, yet it brings consequences. If the host cannot control those consequences, the entire arrangement may break down.

“It is possible that these microorganisms became trapped inside coral cells billions of years ago, and that this contact ended up giving rise to a mutually beneficial relationship,” said Del Campo.

“However, these relationships are not intentional and if they are maintained it is thanks to certain commitments.”

Seen this way, the system is less a charming partnership than an agreement that functions only when both parties remain within certain boundaries.

For the coral, that boundary may be a need for a daily repair period to withstand the costs of hosting microbes.

Sleep and coral survival

The findings indicate that sleep may be older than many scientists had thought. If a coral with no neurons displays a genuine day-night resting pattern, sleep-like cycles may have arisen early in animal evolution, long before brains appeared.

The research also points to a possible reason sleep exists: not as an indulgence, but as a repair mechanism that organisms retain because it is effective.

“We often think that only animals with a brain need to sleep, but all living beings need to repair themselves in some way. Establishing an internal rhythm that facilitates this is a very ancient and successful evolutionary strategy,” said Del Campo.

There is a practical implication as well. Coral reefs face severe pressure from warmer oceans, pollution and other stressors. Restoration programmes frequently grow corals outside the sea before replanting them.

The new findings could influence how such projects are managed, by showing that corals require appropriate day-night cycles to remain healthy.

Rethinking symbiotic relationships

“We now have a better understanding of the physiology of corals, the great architects of marine ecosystems,” said Weller.

The broader message reaches beyond reefs. The study concerns more than coral rest: it considers how life learned to coexist with other life inside itself, and how daily rhythms may have enabled that process.

“This study invites us to rethink symbiotic relationships in nature and value the essential role they have played throughout evolutionary history,” concluded Del Campo.

The complete study was published in the journal Cell Host & Microbe.