Scientists Have Finally Found The First Real Reason We Need To Sleep

Courtesy of Business Insider:

We know we need to sleep. We know our brains and bodies work better after sleep. But what we didn’t know, until now, was why.

Scientists have just reported the first major mechanical reason our brains need to sleep — certain cleaning mechanisms in the brain work better when we shut the brain down. Just like how dump trucks take to the city streets during the pre-dawn hours because there’s less traffic, our brain’s cleaners also work best when there’s less going on.

“This study shows that the brain has different functional states when asleep and when awake,” study researcher Maiken Nedergaard, of the University of Rochester said in a statement. “In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness.”

We’ve known that our brains consolidate memories during sleep and perform other important functions. There are also benefits to the body during sleep — resting allows our muscles, bones, and organs to repair themselves. It also keeps our immune system healthy.

We know that sleep has all of these benefits, but until now we didn’t know any of the specific changes that bring about these sleep benefits.

Charles Czeisler, a sleep researcher at Harvard Medical School in Boston, told Science Magazine’s Emily Underwood that this is the “first direct experimental evidence at the molecular level” for why we need to sleep.

The paper was published in the journal Science on Oct. 17.

Toxic cells

All of our cells accumulate waste while they are working, and these waste products can be toxic. If they aren’t removed they can build up and kill our cells. Throughout the rest of the body the lymphatic system washes these waste products away, but the brain is cut off from these actions because of the blood-brain barrier.


The purple areas are the channels through which brain fluid flows, and the green areas are the glial cells that control the flow of fluid through them.

The team just discovered the brain’s unique trash disposal system last year — the find was published in the journal Science Translational Medicine on Aug. 15, 2012. It works like a plumbing system.

The brain itself is bathed in a special clear liquid called cerebrospinal fluid, which doesn’t mix with the blood and lymph system of the rest of the body. In the study from last year, they found that this fluid travels through special channels and washes the brain out.

There are two types of cells in the brain — the neurons that send signals and the glial that keep them healthy. They found that these glial cells seem to create these cleaning channels around the neurons.

It washes away toxic proteins and removes them from the brain’s circulatory system. They are transferred to the general circulatory system, where the liver can remove them.

Sleeping mice


When mice sleep, fluid-filled channels (pale blue) between neurons expand and flush out waste.

By studying this newfound pathway in mice trained to sleep on a microscope, the researchers found that this system was 10 times more active during sleep than it was while the mice were awake.

They injected the mice with colored toxic proteins to see the system at work — when the mice were sleeping, these toxic proteins were removed from the brain twice as quickly as when they were awake.

In the new study, they found that while the brain is sleeping, the neurons shrink by about 60% and the channels between these cells grow and fill with fluid. The glial cells then activate their pumping system to push the brain’s cerebrospinal fluid through these extra spaces and flush out the area around the neurons.

When we wake, these channels squeeze shut again as the cells plump up, and the cerebrospinal fluid is once again found mostly around the surface of the brain, not deep inside it. While awake, this washing process acts at only about 5% of its performance during sleep.

All of this fluid movement is energy intensive, which is why the researchers think it can only happen effectively during sleep. Normally, all of our brain’s energy is busy doing normal brain activities that support every thing we do — all of our movements, our thoughts, creating memories, and analyzing the signals that come in through our senses. By shutting these processes down, our brains are able to switch into cleaning mode.

Understanding sleep

The toxins that this pathway removes are the kind responsible for neurodegenerative diseases like Alzheimer’s. Understanding this pathway not only helps us understand our need for sleep, and possibly control it better with drugs that turn it on and off, but could also lead to new ways to treat and prevent these diseases.


The buildup of toxic waste proteins causes brain cells to die in Alzheimer’s disease.

In a Perspectives article in Science Magazine about the study, Suzana Herculano-Houzel, a brain researcher at the Universidade Federal do Rio de Janeiro, even suggested that this build up of toxins is what turns on our need to sleep and makes us sleepy.
The mice in the study were woken up after 60 minutes of sleep, so we don’t yet know how the amount or kind of sleep humans get affects the washing process.

While it sounds counter-intuitive, this could even explain why some small-brained animals need more sleep than large-brained animals. For example, bats sleep up to 20 hours a day, while elephants sleep four. Why? Because bigger brains have more space to store these toxins before they build up to dangerous levels and need to be flushed.

Understanding how “brain structure and function changes in the two different states (sleep-wake) suggests that we can start to think about how we can manipulate the two states,” Nedergaard told Business Insider in an email. Manipulations could include ways to put this cleaning system into “hyperdrive” so we could sleep less, but that’s way in the future.


21 thoughts on “Scientists Have Finally Found The First Real Reason We Need To Sleep

  1. BEAUTIFUL blog (and great content). Impressive job on this particular article.

    FYI – just linked this as Related Content to “ADD/ADHD and TIME: 5 Systems Basics” on Expect a few more links too – as soon as I blog-housekeep some of the articles in my Sleep Series.

    Madelyn Griffith-Haynie, CMC, SCAC, MCC
    – ADD Coach Training Field founder; ADD Coaching co-founder –
    (blogs: ADDandSoMuchMore, ADDerWorld & ethosconsultancynz – dot com)
    “It takes a village to educate a world!”

      • GREAT – you are now linked to the HUGE sleep resources “collection of articles” posts – and more (I’m movin’ thru so don’t really track all my updates). Pinging will help me manage my time!!

        BTW – guest posting is an open invitation – your style is similar to my own, so my readers will be able to follow your content fairly easily. Almost anything you’d like to write about would be fair game with the “so-much-more” sort-of-focus to my blog. (If it’s brain-based, & cognition enhancing, its relevant!)

        Personally, I LOVE the way you develop a theme & break down complex ideas into easily understood explanations without “dumbing down.” A rare talent, especially in the science field — frequently those who have gotten used to Journal-publishing conventions, but often because science-geeks tend not to focus on writing skills.

        Just remember to break paragraphs whenever remotely possible for my crowd – most of my readership struggles with focus and reads “from rock to rock.” EVEN tho’ they are frequently up for a long article (as mine are), they’ll run screaming from long runs of words — regardless of what the English teachers might say about my “one-sentence paragraphs,” indents, bullets, headings, etc.

        btw- I thought I was already “following” you – just noted I needed to, so now I AM (even tho’ I’m not a big fan of the WordPress Reader, I like to “follow” to support blogs I like that I want to see “out there” – SEO be damned!)
        xx, mgh

      • Thanks v much, I try to break down subjects as there is so much obfuscation these days. Too many words, too much opinion and floating around ideas instead of getting to the crux of the issues.

        I’d luv to Guest Post sometime, I’ve studied abit of NLP and I myself was diagnosed with a hyperactivity disorder, before ADD was widely known though it was due to my diet. Also diagnosed with cylothmia which I treated myself. If you would ever like to post on my blog you would be more than welcome.

        Same reason as myself, I like to support fellow bloggers who are doing great, honest work x

      • “obfuscation” – one of my FAVORITE words (always brings to mind a great bumper sticker posted above the desk of an ex — “Eschew Obfuscation!”)

        ANY time(s) re: guest posting – inquiring minds would LOVE to hear more about how you self-treated cyclothymia – and not just because my mood swings category really needs content! (apologies if my assumption that “cylothmia” was a typo is incorrect – enlighten me, if so)

        RE Diet & hyperactivity – BIGTIME link (tho’ not causative in ADD-land, diet issues can push someone over the dx line who doesn’t really belong there)

        HEY – what are we both doing housekeeping our blogs on a Sunday morning (for me, anyway – Eastern timezone, US) More to the point, what other to-dos are we both AVOIDING by housekeeping our blogs ::vbg:: I’ll never tell – but mine’s hateful, and this is a lot more fun!
        xx, mgh

      • Its one of my fav words, drop that into a convo and people suddenly start to listen. I’ll work something out in the next couple of weeks and get an article over to you, and you were right to correct my typo! I’ve picked up some techniques along the way but a lot of it was down to perception and resolving inner angst but will go into more detail in the article for you.

        Well its almost 6pm GMT and its about time for me to make dinner, toad in the hole tonight I thinks. With me being on GMT I’ve done all my household chores so no avoiding for me, been a productive old day, I’ve got a tidy house and soon will haven full belly x

      • I *just* finished my breakfast: cheese & tomato omelet – and you’re eating dinner. Ah, the wonders of the i-net. (wouldn’t our great-grandparents be amazed – their missives traveled across the ocean by ship – once Air Mail became available, it was costly, and telegraphs were reserved for emergencies)

        My digs would never be referred to as “tidy,” but at this very moment it looks like I’ve been looted.

        I’m in the middle of my “changing out the seasons” process — making room for all the winter stuff and storing the summer, so everything’s everywhere. And that’s just ONE of the to-dos on my list of chores I hope to accomplish before the work-week begins anew. (My kingdom for a staff and a butler — or A LOT more closet and storage space)

        Looking forward to your article! (no rush – good is worth waiting for). For now, on to MY chores ::groan::

      • Good start to the day with an omelette and my dinner was rather tasty if I must say myself. I think their heads would explode, we live in a golden age with the internet, we are very lucky to have so much info at the end of our fingerprints.

        I said a couple of weeks as I wouldn’t want to let you down on timeframe but hopefully will have something to you before then. Enjoy the chores ha x

      • Protein in the AM stokes the dopamine fires (as well as meds efficacy). Good thing I love eggs!

        re: your article link (tough to read the science shorthand and stay tracked – especially since things were reported as “inversely related”).

        In any case, I knew that Omega 6 fatty acids were GREAT to jumpstart overall cognitive efficiency, & it’s good to have a link to a study that underscores the truth behind “fat and happy” ::grin:: I guess chocolate isn’t the best source for those fats though, huh? Too bad – it’s MY favorite mood elevator.

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  6. I read this news in an article in, hmmm Was it wired or MIT news. Anyway, it’s good to know.
    Thanks for putting this out there.

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