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Dr Karl looks at the latest theories.</p><p class="byline">By <a href="https://www.abc.net.au/profiles/content/s2193276.htm?site=science">Karl S. Kruszelnicki</a></p> <div class="right"> <div id="storyRelatedMedia"> <div id="storyPhotos" class="photo imgBox"><a id="storyPhotosLink" href="https://www.abc.net.au/reslib/201311/r1197142_15458151.jpg"><img id="storyPhotosImg" src="https://www.abc.net.au/reslib/201311/r1197142_15458151.jpg" alt="Sleeping child" class="featureImg"></a><p id="storyPhotosCaption" class="caption">When we sleep, networks in the brain undergo very organised patterns of activity <em>(Source: 4774344sean/iStockphoto)</em></p> </div><script type="text/javascript"></script></div> <div id="related"> <h3>Related Stories</h3> <ul> <li class="expandable"><a href="https://abcmedia.akamaized.net/science/podcast/gmis/gmis20131105.mp3" onclick="if (typeof showAudio == 'function') return showAudio('3883529-mediarss.xml', this, '100%', true, '1197158');" class="audio"><strong>Audio: </strong>Why do we sleep? <span class="source">(Science Online Audio)</span></a></li> </ul> </div> </div> <p>It makes sense that we don't understand difficult and complicated stuff like quantum mechanics and dark energy. But we also don't know simple stuff — such as, why do we sleep? Why do we spend about one third of our lives virtually paralysed, as well as blind and dumb?</p> <p>Sure, we sleep because we are sleepy. But why do we get sleepy? And why after about 16 hours? Why not one hour, or 60 hours?</p> <p>It's not that the brain is resting. In fact, when we sleep, networks in the brain undergo very organised patterns of activity — and use almost as much energy as when we are awake. </p> <p>We also know that during sleep, the brain orchestrates the manufacture of many different types of molecules — proteins, steroids, cholesterol, lipid rafts, human growth hormone, and many more. On the other hand, if you don't get enough sleep (as happens to many shift workers), you suffer increased risk of heart disease and some cancers.</p> <p>But why do we sleep? Almost certainly, it'll turn out that there are many different reasons — all of them correct to varying degrees.</p> <p>Some sleep scientists claim the ultimate purpose of sleep is to prune or weaken brain connections that you have made while awake. This means that the next day, your brain won't be overloaded with irrelevant memories. </p> <p>Other sleep scientists point to research that suggests that sleep replays and consolidates significant memories.</p> <p>Some recent research points out that during sleep, some nerves send their electrical impulses in the opposite direction they normally do when you are awake. We're not too sure why they do this.</p> <p>And other current research says that sleep gives the brain the opportunity to flush out the various metabolic waste products created during the day. </p> <p>If you look at the brain, there's a lot of obvious anatomical stuff — the nerve cells, the glial cells that support the nerve cells, the arteries and veins, and the cerebrospinal fluid (or CSF) that keeps the brain afloat. </p> <p>What's unappreciated is the space in between these structures — the gap, the void, the so-called interstitial space, that is filled with the wondrous liquid called CSF. This interstitial space makes up about 20 per cent of the volume inside your skull.</p> <p>Now, the brain weighs about one-and-a-half kilograms. The trouble is that it's all soft and squishy, with virtually no structural integrity. So normally, the weight of the brain would crush and damage all the structures at the bottom of the brain. So one very important role of CSF is to buoy up, or float, the brain, so that its effective weight is only about 25 grams — and the stuff at the bottom doesn't get damaged by the weight of the stuff above it.</p> <p>At any given moment, there's about 140 millilitres of CSF in and around the brain, and the spinal cord. However, you make about 500 millilitres of CSF each day, so it's always being made — and always being absorbed. This means that you have massive over-capacity to make extra CSF. And most of this CSF flows through the interstitial space.</p> <p>During sleep, the brain cells shrink, and so the interstitial space increases by about 60 per cent in volume. This means that in the case of the brain, when you sleep, the flow rate of the CSF increases by an astonishing 20 times. Suddenly, the waste products of brain metabolism that have built up during the day are whisked away by the raging torrent of cerebrospinal fluid when we are asleep.</p> <p>We know that in some cases, excess quantities of proteins in the brain are linked to various neurodegenerative diseases — for example, the protein beta-amyloid is linked to Alzheimer's disease. These, and many other similar proteins, are normally present in the interstitial space. </p> <p>Purely as a research study, the scientists found that certain drugs could stop this flushing action of the massively increased flow rate of the CSF when we are asleep. As a direct result, this increased levels of nasty chemicals in the brain. </p> <p>Perhaps, with different drugs, they could increase the flushing action of the CSF while we are awake, and reduce the levels of these nasty chemicals — and then we could get de-wasted in the process …</p> <p class="tags"><strong>Tags: </strong><a href="/science/tag/browse.htm?site=science&topic=latest&tag=neuroscience">neuroscience</a>, <a href="/science/tag/browse.htm?site=science&topic=latest&tag=sleep">sleep</a></p> <p class="right top"><a href="#top"><span>^ to top</span></a></p> <p class="date">Published 05 November 2013</p> <p><small><strong>© 2024 Karl S. 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Why do we sleep? › Dr Karl's Great Moments In Science (ABC Science)