Why exactly do human beings need to sleep? It may be hard to believe, but scientists are still debating exactly why we require that about a third of our lives is spent basically “unconscious”.
In the 18 October 2013 edition of the journal Science (subscription required), several articles were published that shined new light on why we need so much sleep.
The New York Times recently reviewed the Science articles, and noted that if you think about sleep in evolutionary terms, it must carry out critical functions for our survival, since not so many centuries ago, people at sleep were likely to be attacked by foreign tribes or eaten by predatory beasts. But even then, they all slept, despite these dangers.
The newer theories proposed in Science, and supported by animal research studies, is that a big reason for sleep is that is the only time we can flush out the metabolic “garbage” that collects in our brains during the day.
Your brain and body can be thought of as factories, and all factories produce both products that you want, but unwanted waste as well.
For example, when you exercise your muscles, you grow them but also produce waste products, such as lactic acid, that your body needs to clean out. Your brain is like a muscle that works especially hard every moment you are awake. Even though it doesn’t move, it uses about 20% of your body’s energy.
At night, your brain can relax a bit. Finally it is not needed to monitor your environment, plan your activities, send orders to your muscles and all that, and during sleep your brain can devote its energy to cleaning out waste products—such as the protein beta-amyloid—which is found in high concentrations in brains with Alzheimer’s disease.
The prevailing theory is that the brain is constantly cleaning itself out, breaking down the waste products in each cell, but the Science research, showed that there is another system at work in the brain—called the “glymphatic” system. It is similar to the lymphatic system, which helps clear the waste products in the rest of our body, but our brains don’t have a lymphatic system. It may have this glympahtic system instead.
Anatomically, the glymphatic system is composed of the fluid spaces between our individual brain cells, about 20% of the brain’s volume. Using sophisticated fluorescent tracers and scanning techniques, the Science researchers found that in mice, these inter-cellular spaces expanded at night, as cerebrospinal fluid flows into these spaces, and washes out the waste products into the cerebrospinal fluid.
Such as system was theorized a century ago, but only recently has the technology been available to carry out the necessary experiments. Glymphatic studies have also been carried out in dogs and baboons, but not yet in humans, where, as you might imagine, such studies are more complicated, and awaiting approval to be conducted.
Neuroscientists suspect that human brains that don’t eliminate the waste products adequately not only don’t function as well (diminished cognition), but may be more prone to degenerative neurologic diseases such as Parkinson’s and Alzheimer’s.
These studies point to another reason that each of us should give priority to obtaining adequate sleep, despite all the other activities that compete for our time. We might think of good sleep as important as a good diet or adequate exercise.
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