Scientists at Imperial College London have discovered that astrocytes — brain cells once regarded largely as support cells — produce rhythmic chemical signals that help synchronise the brain’s master biological clock. The finding adds a surprising new component to our understanding of how the body keeps track of day and night.

Every cell in the human body needs to know what time it is.

Our sleeping patterns, hormone production, metabolism, body temperature and many other biological processes follow an approximately 24-hour cycle.

These cycles are known as circadian rhythms.

At the centre of this extraordinary biological timing system lies a tiny region of the brain called the suprachiasmatic nucleus, or SCN.

It functions as the body’s master clock.

Researchers at Imperial College London have now discovered that cells called astrocytes play an unexpectedly important role in keeping that clock synchronised.

More than support cells

The human brain contains billions of astrocytes.

They get their name from their star-like shape and belong to a broader family of non-neuronal cells known as glia.

For many years, neuroscience concentrated overwhelmingly on neurons.

Neurons transmit electrical signals and form the networks associated with sensation, movement, memory and thought.

Astrocytes were consequently often portrayed as supporting actors — cells that maintained the environment in which neurons could function.

That picture has changed dramatically.

Researchers now know that astrocytes participate actively in communication within the brain.

The Imperial study provides another example.

They appear to be part of the machinery that keeps the brain’s biological clock running properly.

The master clock

The SCN is located in the hypothalamus and contains thousands of specialised cells.

Information about light reaches this region from the eyes.

That allows the brain to align its internal clock with the external cycle of daylight and darkness.

But keeping thousands of cells synchronised presents a problem.

Individual cellular clocks can drift.

For the SCN to function as a reliable master clock, its cells must therefore continually coordinate their timing.

The Imperial researchers discovered a previously underappreciated mechanism helping them do exactly that.

A pulse every few seconds

The team studied brain tissue from mice and observed chemical activity inside astrocytes.

They discovered something striking.

Astrocytes produced rhythmic pulses of calcium.

These calcium signals occurred approximately every 20 seconds.

The pulses then stimulated the release of another signalling molecule: ATP.

ATP is famous as the molecule cells use to store and transfer energy.

But outside cells, ATP can also function as a chemical messenger.

The researchers found that rhythmic ATP release from astrocytes influenced nearby neurons in the SCN.

A chemical metronome

The scientists compare the mechanism to a metronome.

A musician uses a metronome to provide a regular beat that keeps the performance in time.

Astrocytes appear to perform something similar inside the brain.

Their repeated chemical pulses provide timing signals that help neighbouring clock neurons remain coordinated.

Professor Martin Hume, one of the senior researchers involved in the work, described the mechanism as a way in which astrocytes can help organise neuronal activity across the SCN.

The brain’s clock is therefore not simply a network of neurons ticking away independently.

It involves communication between different types of brain cells.

When the metronome is disrupted

The researchers also interfered with astrocyte signalling.

When the rhythmic calcium activity was altered, communication with neighbouring neurons changed as well.

This provided evidence that the pulses were not merely an incidental feature of astrocytes.

They were actively influencing the neural circuitry involved in circadian timing.

The findings suggest that reliable biological timekeeping depends partly on interactions between neurons and astrocytes.

Two clocks working together

An intriguing picture begins to emerge.

Individual cells contain molecular clocks based on cycles of gene expression.

Certain genes become active.

Proteins are produced.

Those proteins then influence the activity of other genes.

The cycle repeats approximately every 24 hours.

But a collection of independent cellular clocks would gradually drift apart without mechanisms keeping them synchronised.

The newly identified astrocyte signalling provides an additional layer of coordination.

There is a slow molecular clock operating over roughly 24 hours — and superimposed upon it are much faster chemical pulses occurring every few seconds.

The brain appears to combine these different timescales to maintain a coherent daily rhythm.

Why circadian rhythms matter

Circadian disruption is more than an inconvenience.

Anyone who has experienced severe jet lag knows how strongly a displaced biological clock can affect sleep, concentration and mood.

Long-term disturbances of circadian rhythms have also been associated with a wide range of health problems.

Shift workers, for example, repeatedly force their behaviour to conflict with the natural light-dark cycle.

Understanding precisely how the SCN maintains synchronisation may eventually help researchers better understand sleep disorders and diseases associated with disturbed biological rhythms.

The Imperial researchers emphasise, however, that this is fundamental biological research.

It does not yet provide a new treatment for insomnia or circadian disorders.

The underestimated cells of the brain

Perhaps the broader significance of the study concerns astrocytes themselves.

For much of the history of neuroscience, neurons occupied centre stage.

That was understandable.

They generate electrical impulses and form the networks through which information travels.

But the brain is not composed of neurons alone.

Astrocytes regulate neurotransmitters, influence synapses, help control blood flow and maintain the chemical environment surrounding neurons.

Research increasingly suggests that they also participate directly in information processing.

Now another function can be added to that list:

helping the brain keep time.

A clock within a clock

The human body contains an extraordinary hierarchy of rhythms.

The Earth rotates once every 24 hours.

Light entering our eyes tells the brain where we are within that cycle.

The SCN translates that information into biological time.

Individual cells maintain molecular clocks.

And inside the brain’s master clock, astrocytes produce rapid chemical pulses that help keep thousands of neurons synchronised.

The mechanism operates continuously and invisibly.

We do not hear it.

We do not feel it.

But while we work, eat, sleep and wake, a microscopic chemical metronome is beating inside the brain.

And it helps ensure that the rest of the body knows what time it is.

Source: Based on “‘Chemical metronome’ helps the brain to keep time,” published by Imperial College London on 3 December 2024. The underlying research investigated rhythmic calcium and ATP signalling by astrocytes in the brain’s suprachiasmatic nucleus and was published in The EMBO Journal.