Why do we sleep? Scientists have long wondered why humans spend approximately a third of their lives asleep – over 25 years for the average American. It’s especially curious from an evolutionary perspective. While sleeping, we’re not feeding or foraging, we’re not socializing, we’re not childbearing, and we’re not mating. And worse yet, we are incredibly vulnerable to predators. After all, a sleeping person is an unconscious person. Yet sleep has persisted through evolution, and scientists believe sleep must have some essential biological function. A notable sleep pioneer and researcher, Allan Rechtschaffen, once said that “If sleep does not serve an absolutely vital function, then it’s the biggest mistake the evolutionary process has ever made.”
The glymphatic system: the brain’s specialized plumbing system active during sleep
After decades of research, scientists are finally beginning to understand the multitude of reasons why we all (should) spend 7-9 hours a night sleeping. And one particular function of sleep has gained significant traction: the glymphatic system – a specialized plumbing system that activates during sleep to help rinse away toxins and other metabolic waste from the brain.
All cells in the body create waste and other metabolic byproducts. In a majority of tissue, lymphatic vessels collect and carry away “sewage” to the bloodstream and into the liver and kidneys to excrete it. But the brain lacks a traditional lymphatic system. So, how does the brain dust itself off?
Nearly 10 years ago, researchers from New York State’s University of Rochester Medical Center found a striking difference in the brains of sleeping versus awake mice. Researchers used a powerful microscope to track the movement of a specialized fluid, cerebrospinal fluid (CSF), that cushions and protects the brain from bumping into the skull. When the mice were sleeping, neuroscientist Maiken Nedergaard and colleagues found something unexpected: a surge in the amount of cerebrospinal fluid that was recycled into the brain. It was as if the floodgates opened and the cerebrospinal fluid helped shower the brain. But when the mice were awake, the gates shut and only trickles of cerebrospinal fluid seeped through into the brain – as if there was a drought.
This system, now known as the glymphatic system, operates similar to a dishwasher’s plumbing system. During sleep, the space between brain cells balloons to allow cerebrospinal fluid to flow through dedicated channels and crevasses. Through pulses of pressure from arteries, these channels allow cerebrospinal fluid to shower brain cells and sweep up waste that is then transported out of the brain toward to the system’s main outlet, the lymphatic vessels in the meninges. The discovery of this previously hidden “glymphatic system” answers a fundamental question about how the brain keeps clean.
The glymphatic system and deep sleep
Most extraordinary, researchers found that the glymphatic system’s specialized plumbing and waste-management is most efficiently active during periods of slow-wave activity commonly found during deep sleep – the same type of sleep scientists have found to be important for a myriad of restorative functions and memory consolidation.
Unfortunately, deep sleep significantly reduces and becomes less electrically “intense” as we age. In addition to a reduction in deep sleep, hallmarks of sleep changes in older adults after the age of 60 include an increase in sleep fragmentation, and less total sleep. Poor sleep as we age may not be a normal phenomena that we should just accept as a natural process caused by aging. According to Dr. Matthew Walker, a UC Berkeley neuroscientist, “that older adults simply need less sleep is a myth. Older adults appear to need just as much sleep as they do in midlife, but are simply less able to generate that (still necessary) sleep.”
According to Nedergaard, our brains shrink as we age and we produce less cerebrospinal fluid – two processes that may contribute to a reduction in the strength of the glymphatic system. Scientists also believe that the inability to generate sleep as we age may be tied to impairment of the glymphatic system and even certain diseases. For example, glymphatic impairment has been linked to a range of neurological diseases included Parkinson’s, ALS, stroke, hypertension, and diabetes. This is not to say that poor sleep causes these diseases – most research, thus far, has been purely correlational. The most robust finding, however, has been the association between poor glymphatic functioning, sleep, and Alzheimer’s disease.
Sleep, the glymphatic system, and Alzheimer’s disease
A hallmark of Alzheimer’s disease is a build-up of proteins that, when turned into plaques, are toxic and can cause degeneration to neurons. These proteins, known as amyloid-beta proteins, are commonly found in healthy people but are naturally created and removed at near-equal rates. When this balance between equal-parts creation and removal is thrown off, the proteins may build up, spread, and clump into plaques into certain brain regions over time. New scientific clues show that sleep deprivation may be one way that this protein balance becomes out of whack. Researchers found that just 1 night of sleep deprivation significantly increases the amount of amyloid-beta protein in the brain. Recent studies show that lack of sleep in middle and old age may be associated with cognitive impairment and subsequent dementia. A study published in the journal Nature Communications found that persistently sleeping 6 hours at age 50, 60, and 70 was associated with a 30% increased dementia risk compared to those sleeping 7 hours a night. It is important to remember that this research is still in its infancy and more controlled, long-term studies are needed to replicate and confirm the association between sleep, glymphatic impairment, and chronic diseases.
Studies are already underway to potentially harness the power of sleep, increase glymphatic activity, and perhaps even halt age-related diseases like dementia. The future of sleep research is an exciting one – filled with the possibility of using sleep as our superpower to improve lives for millions worldwide.
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