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The essential "brain-cleaning" function of sleep

Everyone knows the horrible feeling of sleep deprivation, or how terrible you feel after the disruption to your normal sleep schedule by jet-lag. Lack of sleep has been shown to impair cognitive function - making it harder to think - and sleep disturbances have been linked to several neurodegenerative diseases. It is well known that during normal circumstances our sleeping patterns follow a 24-h “internal clock”, which is strongly regulated by our bodies. Sleep, it seems, is something very important, as it is tightly controlled by our biology every day and we become slow and sick if our sleep is disturbed.
But why do we sleep?

Why we sleep has been an open question since antiquity, documented at length by Greek and Roman philosophers, who theorised that dreams came from the Gods. Sleep and dreams were given practical meaning in everyday life through dream oracles and practical dream interpretation manuals (PMID: 19014776).
More recently, several theories have been proposed regarding why we sleep, including the build-up of a “need to sleep”-substance during wakefulness that is removed during sleep. The answer, however, may be a more practical one.

Have you heard of the glymphatic system? No? 
Not to be confused with the lymphatic system, the glymphatic system was discovered in 2012 by Professor Maiken Nedergaard and coworkers at the University of Rochester (PMID 22896675), and is also known as the glymphatic clearance pathway. It is a system that functions as a waste clearance mechanism for the brain and spinal cord. The glymphatic system works through the contraction and expansion of the space between cells in the brain, which allows fluid to “flush” out waste products. The neuronal cells of the brain are highly sensitive to their environment, and it is therefore of great importance that any garbage or waste is removed quickly and efficiently.
The discovery of the glymphatic system provided an answer to the long-standing question of how the brain manages the removal of waste, extra fluid and signalling proteins. In the rest of the body, the lymphatic drainage system as well as the immune system deals with waste removal, however, the brain and spinal chord are sensitive structures which are closed off from this cleaning system to prevent damage from inflammation and other defensive “cleaning” processes. 

So what does this have to do with sleep? 
Professor Nedergaard and coworkers have shown that the glymphatic “flushing” of the brain is the most active during sleep, and that the expansion of the space between the cells in the brain is at its greatest during sleep to support this “cleaning” flow (PMID 241369709). Thus the restorative properties of sleep - which is evolutionarily conserved across all animal species - are most likely caused by the removal of waste products which accumulate in the awake brain. All that thinking and the executive function performed during the day needs to be cleared out at night!
Even more recently, during 2020, Professor Nedergaard's team have shown that glymphatic clearance in the brain is dependent on the circadian rhythm, i.e. that it follows the natural internal process that regulates our sleep-wake cycle (PMID: 32879313). 

So, the feeling of waking up with a clear head after a good night’s sleep may not only be a measure of being rested but also that your brain has had a good “scrub” and is now squeaky clean and ready to take on a new day. 
Here at Saga Sleep we think of sleep as a health activity, allowing the body time to rest and recover after a day’s work, but also to support brain health and activity. Try our app to help establish good sleep hygiene and get a good night’s sleep every night - for you and your family.