The Night Shift
At any given moment, the brain’s approximately 85 -100 billion neurons (different sources offer a variation on the exact figure) are engaged in countless conversations, through its incredible network. Collectively, trillions of connections are exchanging signals, generating an estimated quadrillion or more neural events every second. This extraordinary activity continues even while we sleep, as the brain regulates bodily functions, consolidates memories, processes emotions, and carries out essential maintenance tasks.
Most of us think of sleep as a time when the brain switches off. However, the brain is a genius permanently at work. While our conscious minds are resting, a remarkable team of workers begin a night shift deep within the brain. These workers are called glial cells.
For many years, neuroscientists believed that glial cells existed mainly to support neurons. So, neurons have received most of the attention and the glory, while glial cells were thought of as little more than the brain’s housekeeping staff. Research over the past few decades has completely changed that view, elevating them to greater prominence.
Today, scientists know that glial cells are critical in maintaining the brain’s environment by regulating communication between neurons, supporting learning and memory, defending against infection, managing inflammation, and helping clear up waste products that accumulate during the day.
One of the most fascinating discoveries in recent years is that many of these housekeeping functions become especially active during deep restorative sleep (delta wave).
The Secret to Health is the Removal of Waste
Throughout the day, the brain is incredibly busy. Every thought, memory, emotion, movement and decision generates metabolic activity. As a result, waste products naturally build up between brain cells, in the same way as rubbish accumulates in a busy city.
Unlike the rest of the body, the brain does not have a traditional lymphatic drainage system to remove this waste. Instead, it relies on a specialised network known as the glymphatic system, a name that combines ‘glial’ and ‘lymphatic’.
This system is largely coordinated by astrocytes, a type of glial cell that surrounds blood vessels throughout the brain. During sleep, particularly during deep delta-wave sleep, these astrocytes help facilitate the movement of cerebrospinal fluid (CSF), through brain tissue. This fluid washes the spaces between the neurons, helping to remove metabolic waste and potentially harmful proteins.
In a landmark study published in Science (a leading international peer-reviewed academic journal) in 2013, researchers led by Maiken Nedergaard discovered that the spaces between brain cells expand during sleep. This expansion allows cerebrospinal fluid to flow more freely, dramatically increasing the brain’s ability to clear waste products. The findings provided compelling evidence that sleep serves as a critical biological cleaning function.
Among the substances removed during this process are proteins such as beta-amyloid, which have been linked to neurodegenerative conditions including Alzheimer’s disease when they accumulate excessively over time.
But waste clearance is only part of the story…
The Genius of the Glial Cells
There are as many glial cells as there are neurons, so there are 85 billion of these geniuses, divided into four groups each with their own specialisms and tasks. These include, but are not limited to, supplying oxygen and nutrients to neurons, structurally supporting neurons (holding them in place), destroying pathogens, removing dead neurons, and circulating cerebrospinal fluid which protects the brain from injury.
The glial cells, known as microglia, act as the brain’s immune system. These cells constantly monitor the brain’s environment, identifying damaged cells, debris and potential threats. During periods of healthy sleep, microglia continue their surveillance and maintenance work, helping to preserve the integrity of the neural networks.
Researchers have also discovered that glial cells contribute to the maintenance and remodelling of synaptic connections – the communication points between neurons. This process helps the brain strengthen important connections while pruning those that are no longer needed. In many ways, sleep allows the brain to review the day’s experiences, reinforce learning and maintain efficient neural pathways.
The glial cells called oligodendrocytes, are responsible for producing myelin, the protective fatty insulation that surrounds nerve fibres. Healthy myelin allows electrical signals to travel quickly and efficiently at incredible speeds across the brain and throughout the nervous system. Emerging evidence suggests that sleep also supports processes involved in myelin maintenance and repair.
Sleep a Vital Pillar of Mental Health
Taken together, these findings paint a very different picture of sleep from the one many of us grew up with. Sleep is not a passive state of rest. It is an active period of restoration, repair and maintenance. While we sleep, glial cells are hard at work regulating the brain’s internal environment, clearing waste, supporting immune function, maintaining neural connections and helping preserve the delicate systems that allow us to think, feel and function.
This is one reason why poor sleep can affect so many aspects of our lives. When we consistently miss out on deep restorative sleep, we are depriving the brain of the time it needs to perform these essential housekeeping tasks, which are critical to our survival, hence the primacy of the glial cells.
Far from being idle support cells, glial cells are among the brain’s most important caretakers. And every night, while we rest, they quietly begin their night shift – working behind the scenes to keep one of the most complex structures in the known universe healthy and functioning for another day.
References
1. Xie, L., Kang, H., Xu, Q., et al. (2013). Sleep Drives Metabolite Clearance from the Adult Brain. Science, 342(6156), 373–377.
2. Maiken Nedergaard and colleagues’ work on the glymphatic system.
3. Beth Stevens. Research on microglia and synaptic maintenance.
4. R. Douglas Fields. The Other Brain.
5. Matthew P. Walker. Why We Sleep.
6. Stephen W. Porges and related work on nervous-system regulation during restorative states.