Why do we sleep?

 A study funded by the National Institute of Neurological Disorders and Stroke (NINDS) discovered that the brain may clear toxins that have built up during the day.  We already know that sleep is fundamental in safeguarding memories and now it seems that the brain may increase the space between brain cells allowing the fluid to flow through rapidly. This is part of the glymphatic system which controls the flow of cerebrospinal fluid through the brain.
  Researchers first injected dye into the CSF of mice to see how it flowed through their brains and monitored their electrical brain activity. They compared the flow rate of when they were unconscious to when they were awake and the later was significantly slower. It was then theorised that the distance between brain cells changed between unconscious and conscious states. This was tested by inserting electrodes into the brain of the mice to measure the space between the brain cells. It was found that the space increased by 60% when the mice were unconscious, allowing CSF to flow more freely.
Stained rat brain cells

Stained rat brain cells

  Brain cells called glia control the glymphatic system by shrinking or swelling. The hormone noradrenaline is known to control cell volume and was discovered to be less active during sleep. This was found by giving the mice drugs which blocked noradrenaline, this subsequently induced unconsciousness. The hormone is usually released when the brain needs to become more alert, for example, in response to fear.
  Other studies have proposed that toxic molecules collect in the space between brain cells. When the protein beta-amyloid (associated with Alzheimer’s disease) was injected into mice, it was shown that there was an accumulation of the protein when they were sleep deprived. This suggests that the glymphatic system is key in clearing the toxic molecules.
  The study carried out may be very helpful in understanding the clearing of toxins and that the glymphatic system may need to be targeted in order to treat certain neurological diseases such as Alzheimer’s.
Note: The research was carried out at the Nedergaard Lab, University of Rochester Medical Center and was published in Science.

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