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What Do Our Brains Do While We Sleep?

by Dr. Reut Gruber
Nov 11 • 4 min read

In her recent award-winning album, Billie Eilish muses “When we all fall asleep, where do we go?” In a subsequent interview, she added “The album for me is what happens when you fall asleep…all things that don’t have an explanation. […] We lay there and suddenly we are awake and it has been 8 hours. This does not make sense to me.”

Sleep remains a mystery in many ways, but recent fascinating discoveries have allowed us to better understand this daily activity that has been puzzling people for millennia. Here is what we know about what happens when we lay in bed sleeping.

Maintenance and Repair

Think about a car that travels long distances every day. This machine will need to be regularly cleaned and repaired. Your brain is like that car, helping you travel through life. It’s on the go every minute you are awake. This is a lot of work. Just as intense use of a car can cause a lot of wear and tear, the daily use of the cells in your brain (neurons) causes damage within them.

One type of damage to our cells and tissues is related to the massive amount of oxygen the brain uses every day. About 20% of the oxygen we take in is used by the brain to power the roughly 86 billion neurons and ~250–300 billion cells that support them. The breakdown related to oxygen use is called “oxidative stress”. This stress arises when there are more oxidants (molecules that attack our brain cells) than antioxidants (molecules that protect our brain cells) in our brain, resulting in damage to our brain cells. This damage can impair the work of our entire body. It is critical that our brain cells be repaired on a regular basis, by the end of each day. Much of this “neural repair” happens during sleep.

While we sleep our neurons are restored and repaired and the level of oxidative stress in our brain is reduced. This process protects our neurons and prepares us to pursue our dreams when we are awake.

Sleep: Your Brain’s Waste Management

Another important activity during sleep is waste management.

The brain is one of the most active organs in the body. Think about all the things you do every day. Even on a lazy day you still do plenty – you breathe, you move, you eat, and you think. Each of these activities requires energy, which is produced by your body’s cells. When brain cells consume high amounts of energy, they spit out a lot of debris that floats around the brain. This debris consists mostly of leftover proteins, such as the protein called beta-amyloid, which can form clumps that are toxic to the brain. Sleep is the time for cleaning up this mess.

Every night, during the deepest sleep stage known as slow-wave sleep, our brains clear the “neurological debris” created during the day’s activities. Long-term accumulation of this neural debris is related to brain disorders such as dementia and Alzheimer’s disease.

Replaying Our Daytime Experiences

Another job the brain does each night is storing the new experiences we gathered during the day.

We go through a variety of experiences on any given day, both new & routine, that range from thrilling to mundane. Which of these events are we going to remember? This depends on what happens in our brain when we sleep.

For an experience or event to be remembered, it has to be replayed in our sleep. Each night our daytime experiences are repeated (replayed) during our deep sleep. By replaying and repeating our experiences, our brain makes their memory traces stronger and moves them to the part of the brain that serves as our “long-term storage”. We can then access the memory in the years to come.

Our sleep is in charge of “following up” on our daytime experiences, and cleaning up the mess created as we go through our day. It is the time for repairing the damage that occurred in our brain cells when we were active, for managing the waste produced by them, and for storing and organizing experiences.

So much is happening when we lay in bed sleeping. Just thinking about it is enough to make you tired!

Author Bio

Reut Gruber is a scientist, psychologist and sleep expert. She is an associate professor in the department of psychiatry at McGill University and director of the Attention, Behaviour and Sleep lab at the Douglas Mental Health University Institute; Diplomate of the American Academy of Cognitive Therapy (ACT) USA; Licensed Psychologist, l’Ordre des Psychologues du Québec (OPQ), QC, Canada.

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