Fatigue is a part of life, and therefore the need to sleep is a part of life. The relationship itself is pretty simple, but the reasons humans (and all animals) must sleep remains a complex mystery. Even prominent neuroscientist Jeff Iliff asked in an October 2014 Ted Talk: “Do any of us really understand what it’s all about?”
Opinions on sleep vary greatly. For some, sleep is welcomed relief; for others, it is a waste of 6, 7, or 8 hours. Thomas Edison once said, “sleep is a criminal waste of time and a heritage from our cave days,” while British Prime Minister Margaret Thatcher claimed sleep was for “wimps.”
Even the scientific community wants more answers to why humans sleep and how much they should sleep. Sleeping for fewer than six hours has been linked to early death. But a February 2015 article in the Huffington Post also identified eight health risks of sleeping too much, which range from increasing the risk of depression to causing a 1.3 times greater risk of death. The same 2010 study that uncovered this link also noted that sleep’s “underlying mechanisms, interactions, and long-term effects are still poorly understood.” The mystery becomes even more so when you consider how risky sleeping on the open savanna was for early humans. “Sleep is such a dangerous thing to do, when you’re out in the wild,” Maiken Nedergaard, a Danish biologist who has been leading research into sleep function at the University of Rochester’s medical school, explained to the New York Times’ Maria Konnikova. “It has to have a basic evolutional function. Otherwise it would have been eliminated.”
So what gives?
Humans spend around one-third of their lives sleeping. That means if you live to the age of 90, and sleep an average amount of 8-hours per day, you will have spent 32 years of your life asleep. As Russell Foster, a professor of Circadian Neuroscience and the head of the Department of Ophthalmology at the University of Oxford, noted in a 2013 Ted Talk, that fact alone tells us “that sleep at some level is important.” Yet scientists have not quite figured out all the biological ins and outs of why sleep is so important. Sure, sleep is restorative, and sure sleep helps humans avoid wasting essential calories, but there must be more to the story.
There are two possible, and potentially interrelated, explanations circulating the scientific community. Foster’s theory argues that sleep is essential to brain processing and memory consolidation, and by depriving yourself of sleep, you impair your ability to “come up with novel solutions to complex problems.”
In his talk, Iliff described a very different theory. It’s not glamorous; he postulated that the main reason the brain needs sleep is for waste removal. However unglamorous it is, this theory does have enormous implications for our understanding of Alzheimer’s disease.
“We’ve found that sleep may actually be a kind of elegant design solution to some of the brain’s most basic needs, a unique way that the brain meets the high demands and the narrow margins that set it apart from all the other organs of the body.”
Here’s what Iliff really means:
Every cell in the body needs access to nutrients, and throughout the body, the circulatory system sends “blood vessels to supply nutrients and oxygen to every corner of our body.” And just as cells need nutrients, they also need waste to be removed. The body’s lymphatic system takes care of that task everywhere but in the brain. Even though “the brain is this intensely active organ that produces a correspondingly large amount of waste,” there are no lymphatic vessels. This is largely because there is no room for another set of vessels as the brain is enclosed in the rigid skull.
Instead of a second network of vessels, surrounding the brain is a “large pool of clean, clear fluid called cerebrospinal fluid,” which is pumped through the brain along the outside of blood vessels to flush out waste from the spaces between cells. “This really clever way to repurpose one set of vessels, the blood vessels, to take over and replace the function of a second set of vessels, the lymphatic vessels, to make it so you don’t need them,” Iliff explained.
His research shows that this flushing only occurs “in the sleeping brain.”
What’s the link to Alzheimer’s?
The waste needed to be cleared out is amyloid-beta — a protein that is made in the brain all the time. Scientists believe that a buildup of amyloid-beta is one of the key steps in the development of Alzheimer’s. According to Iliff, recent studies have shown that among patients who have not yet developed the disease, worsening sleep quality and shorter sleep duration are associated with greater amyloid-beta build up. “While it’s important to point out that these studies don’t prove that lack of sleep or poor sleep cause Alzheimer’s disease, they do suggest that the failure of the brain to keep its house clean by clearing away waste like amyloid-beta may contribute to the development of conditions like Alzheimer’s,” he concluded.
For more information, check out Iliff’s Ted Talk below.
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