Synaptic pruning: why your brain shrinks when you sleep
From early childhood to adulthood, your brain shrinks when you sleep. This completely natural process is called synaptic pruning and also refers to the strengthening of brain tissue as the process occurs.
You can think of it as your brain’s way of learning and storing memories while regenerating itself for when you wake up the next morning.
What are synapses?
Within your nervous system, there are neurons, which are the building blocks of the brain. Neurons receive sensory input from the outside world, which they convert into signals and send to our muscles. Synapses connect each neuron to the next; without synapses, neurons would not be able to function.
If neurons are messengers, synapses are the channels that neurons use to relay those messages in the brain. Synapses begin developing when you are a baby. At that stage, it’s called synaptogenesis. This is to give you an excellent foundation to begin learning, making connections between what you see with your eye and to retain information.
What is synaptic pruning?
Consider the brain as a massive warehouse made up of different departments. Each department commands certain roles within the body. For example, there’s the respiratory department that keeps the heart and lungs functioning. Then there’s the digestive department that processes food and the lymphatic department that controls the release of hormones in the glands. The warehouse requires daily maintenance to keep it clean and ensure that all departments are working efficiently.
Like the warehouse, your brain needs daily maintenance and uses synapses as a cleanup function. As you grow, you lose synapses while you sleep so that you can reset your brain to learn new information and process new experiences. Synaptic pruning also makes way for high-quality connections to support complicated mental functions. The brain is like plastic, easily stretchable which is important as you grow.
The brain shrinks by up to 20% when synaptic pruning occurs, showing how synapses make up a large part of the neurological system. The brain will return to its normal size after pruning. If your brain does not perform synaptic pruning during sleep (synaptic homeostasis), synapses could break down, become overburdened and burn out.
How the brain prunes its synapses
Toddlers experience the most synaptic pruning. That’s why they need consistent mental stimulation. Exposing children to learning tools and information from as early as possible promotes synaptic pruning happen on a greater scale so they absorb more information.
Scientists have discovered that as you grow older, your personal life experiences affect synaptic pruning. The more you use a synapse, the stronger it becomes while weaker ones are lost. All this continuously occurs throughout the day but is especially heightened during sleep as part of the regenerative function.
Between the ages of 2 and 10, pruning is at its peak. 50% of your synapses will be shed while in your teens, as pruning begins to slow and stabilise. As you hit your twenties, pruning takes place in the prefrontal cortex. This part of the brain is responsible for personality, critical thinking and decision making.
When you reach your late twenties, your brain no longer shrinks during sleep as pruning effectively ends.
Why does synaptic pruning only occur during sleep?
Studies have shown that temporary synapses developed during the day are pruned at night so that your brain can rest from the complex process. The synapses which remain are core memories which are essential for you to remember.
Synaptic pruning occurs in the rapid eye movement (REM) stage of sleep. Research conducted by Pecking University in China and the University of York in the UK has found that REM sleep has multifaceted functions in brain development, learning and memory consolidation by selectively eliminating and maintaining newly formed synapses.
During REM sleep, your eyes dart back and forth as your muscles relax and temporary sleep paralysis sets in (a natural process to prevent you from acting out your dreams). As your brain activity increases, so too does synaptic pruning.
The connection between sleep disorders and synaptic pruning
According to a study by Oxford University in 2017, there is a link between circadian rhythm sleep disorders and synaptic pruning. The circadian rhythm regulates the body’s internal body clock, initiating wake and sleep cycles. It is affected by light and dark.
The key features of circadian rhythm sleep disorders involve either difficulty falling asleep, waking up during the sleep cycle or waking up too early and being unable to fall back to sleep.
The most common circadian rhythm sleep disorders are:
- Delayed sleep-wake phase disorder
- Irregular sleep-wake rhythm disorder
- Jet lag disorder
- Non–24-hour sleep-wake rhythm disorder
- Shift work disorder
These disorders are characterised by a breakdown in neurons exchanging data which develops into sleep disruption, impaired cognition and memory. This can be linked to inefficient synaptic pruning.
What happens when pruning is disrupted?
Just like the warehouse with all its different functions running simultaneously, a problem can occur, leading to a glitch which affects the whole system.
According to neuroscientific research, over-pruning has been linked to mental illness. A study conducted on the brain matter and DNA of over 100,000 people showed how they have a certain gene which produces over-pruning. Schizophrenia has shown a correlation to over-pruning as people with the illness tend to have a genetic mutation which makes their brains prune more than necessary.
Autism has also been found to occur as a result of under pruning. Instead of shedding synapses during sleep, the brain neglects this function, causing a sensory overload. This overload shows up as the brain becoming ultra-sensitive to external stimuli and over firing. People with autism sometimes become more sensitive to light, sound and noise.
The future of synaptic pruning
It might seem strange that the brain would shrink and shed parts of itself to develop and work efficiently, but synaptic pruning has provided researchers with ideas on how they can help people with mental disorders. It has also provided insight into the importance of childhood education and mental stimulation in the early years.
In future, neurological specialists could be able to use their findings to create therapies and medication to control pruning. In the case of over or under pruning, researchers aim to target the synapses and provide improved treatment for schizophrenia and autism. However, more research is still needed to form definite conclusions of how synaptic pruning affects autism and schizophrenia as well as other mental disorders.
Ideally, synaptic pruning is part of human growth and development. Old and weak synapses are lost during pruning so you can welcome new experiences, absorb new information, learn and adapt the next day.