Why Do We Sleep? The Brain’s Waste Disposal System

Brain Pic

How does sleep help this fellow’s tubular brain remain healthy?

“Okay, smart guy so why do we sleep then? Bet you can’t answer that one…”

“Well… How long have you got? There’s a fair bit of content to co- Hey! Don’t run away! It won’t take me that long, honest!”

Sleep is complex and despite the intuitive explanation for why we drift off on a daily basis, we are still in our infancy when it comes to understanding how and why this occurs. We know that for the most part a good night’s sleep is refreshing and important for paying attention at work, school, or carrying out most tasks.However, how sleep keeps us functioning is less black and white.

There is no one reason why we need to sleep. For example, it maintains us mentally and physically in a number of ways, including:

  • Memory consolidation
  • Growth
  • Mental health
  • Brain maintenance

We could spend a lot of time covering all of these in considerable detail, but for now we’ll look at brain maintenance. Brain maintenance may seem like a broad category but it fits into our recent developments in understanding how the brain deals with waste materials.

The Brain’s Waste Disposal System

All cells in the body produce waste products of some description. This may be as a result of metabolic functioning, protein synthesis, and cell death, and these are filtered out via the lymphatic system.

The brain, unsurprisingly, is no different. However, until recently it was not clear how the brain solved this waste management problem. That was until the recent discovery of the glymphatic system.

Glymphatic System: The Brain’s Clearance System?

So what is the glymphatic system, and why should we care? Well firstly, its discovery has been hailed as important step in understanding major neuropathologies such as Alzheimer’s disease. Although, there’s a while to go before we should get too excited about this.

As mentioned above, the cells within your body work and produce potentially toxic waste products. The body has an effective way of getting rid of these through multitudinous lymphatic vessels. Yet, until recently it wasn’t known how the central nervous system, that is the spinal cord and the brain, manage to remove their waste products.

In a similar way to the lymphatic system, it has been proposed than the glymphatic system works by removing waste products from the central nervous system (e.g. spinal cord and brain). This is achieved, in part, by the exchange of solutes or waste products between cerebrospinal fluid (CSF) and interstitial fluid (ISF).

The system is comprised of a series of channels which appears to “piggyback” off blood vessels within the brain. It is argued that the movement of arteries, as a result of blood-flow, helps to move the cerebrospinal fluid through the brain (CSF). This enables the movements of waste between interstitial fluid (ISF) which bathes cells and CSF which runs alongside blood vessels in the brain. However, the glymphatic system also includes the presence of specific glial cells, known as astrocytes, alongside these channels. The function of the astrocytes is to facilitate the movement of particles between CSF and ISF. Without their presence, the movement of waste would be too sluggish to respond appropriately to the changing demands of the hungry organ we call the brain. It is the presence of glial cells, and the similarities to the lymphatic system, which give this glymphatic system its name.

Waste Disposal While You Sleep

How does all of this relate to sleep? Well, a recent collection of studies has provided some evidence that sleep may facilitate the movement of toxins from the brain via the increased activity of the glymphatic system.

One particular toxin produced as a waste product of brain cell activity is called amyloid beta. This toxin is linked to severe progressive diseases such as Alzheimer’s, and there is considerable research trying to work out how to mitigate its destructive effects. Last year, a group of scientists lead by Professor Maiken Nedergaard at the University of Rochester Medical Centre found that the brain’s drainage of amyloid beta was increased during sleep and anaesthesia-induced unconsciousness compared to during wakefulness in mice. More specifically, it seems that the capacity of the brain to flush out such harmful toxins is enhanced due to the area between cells, interstitial space, expanding by as much as 60 per cent. This seemingly enables a greater opportunity for such toxins to be removed via the glymphatic system.

In addition, the same group also found earlier this year that this process of washing harmful toxins out of the brain during sleep is enhanced based on the position of the mice studied. It was found that the removal of amyloid beta was enhanced when the mice slept on their side rather than their back. Interestingly, in both cases, it seems that anaesthetised mice showed the same effects as sleeping mice for the removal of amyloid beta and benefit of sleep posture.

This suggests that sleep may help with the maintenance of our brains by enhancing the waste disposal system we have in place. It may also provide some evidence as to why dementia and serious psychiatric illnesses are associated with sleep disturbances prior to the emergence of symptoms. However, this is all speculative and uncertain at this point. For a start, these studies have only been conducted on mice, and due to the nature of the experiment it is not likely that they would be conducted on humans any time soon. That’s not to say that the same mechanism might not be occurring within humans, but it is rash to jump the gun and assume it is.

Regardless, it will be exciting to see how research in this area advances and what it can teach us about the importance of sleep.

Inquisitive Tortoise

References:

Lee, H., Xie, L., Yu, M., Kang, H., Feng, T., Deane, R., … & Benveniste, H. (2015). The effect of body posture on brain glymphatic transport. The Journal of Neuroscience, 35(31), 11034-11044.

Xie, L., Kang, H., Xu, Q., Chen, M. J., Liao, Y., Thiyagarajan, M., … & Takano, T. (2013). Sleep drives metabolite clearance from the adult brain.science, 342(6156), 373-377.

Further Reading:

https://www.urmc.rochester.edu/labs/Nedergaard-Lab/projects/glymphatic_system (From the horse’s mouth; a better description of what I’ve tried to convey in this post!)

Image Credits:

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Filed under Psychology, Sleep Science

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