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- Humans spend about a third of their lives sleeping, but scientists don’t quite understand why.
- There are four dominant theories: the theory of inactivity, the theory of conservation of energy, the theory of restoration and the theory of brain plasticity.
- One thing is certain: not getting enough sleep can have disastrous health consequences, ranging from a weakened immune system to depression.
It’s strange when you think about it: human beings spend almost a third of their lives stretched out with their eyes closed, dead to the world. But we have a biological need for sleep because it offers many unbeatable benefits; it gives us more energy, reduces stress and strengthens the immune system. While we sleep, our body also repairs cells and adjusts hormone levels.
“Sleep affects nearly every tissue in our body,” sleep expert Dr. Michael Twery writes in a blog post for the National Institutes of Health. “It affects… our immune system, appetite, breathing, blood pressure and cardiovascular health.”
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On the other hand, not getting enough sleep can have disastrous consequences on our well-being. Drowsy people are much more likely to make bad decisions and be involved in traffic accidents. And chronic sleep deprivation can weaken the immune system and increase the risk of developing depression.
These are all compelling reasons to get fired, but they don’t completely explain Why humans spend eight hours unconscious and paralyzed each night. As noted by sleep scientist Allan Rechtschaffen said once“If sleep doesn’t serve a vital function, that’s the biggest mistake evolution has ever made.”
For decades, scientists have attempted to solve the mystery of why humans sleep, with four possible theories leading the way: inactivity theory, energy conservation theory, restorative theory and the theory of brain plasticity.
Sometimes referred to as the “adaptive” or “evolutionary” theory, the inactivity theory was one of the first attempts by scientists to explain sleep, in the 1920s, before scientists started laboratories of the sleep or discovered rapid eye movements. The idea is that for animals, night is a vulnerable time, when darkness makes it difficult to move around safely or avoid predators. Staying still and quiet was a good way to avoid danger until morning. Over the course of evolution, the theory goes, this strategy eventually morphed into what we now call sleep.
The inactivity theory has one glaring problem, however: falling unconscious at night would have increased vulnerability and made it nearly impossible to react to danger.
Energy conservation theory
This theory suggests that the main function of sleep is to reduce the amount of energy an animal needs and the amount it uses. For most of us today, snacking is usually a simple task, but for ancient humans, finding food required a ton of time and effort. Sleeping through the night – a time when hunting was difficult and dangerous anyway – was a good strategy to save energy.
This theory is supported by research which shows that when humans sleep, their metabolism slows down by approximately 10 percent (this figure is higher in other species). For example, body temperature and caloric needs drop while we sleep. Many researchers consider energy restoration theory to be an integral part of inactivity theory.
After a long night’s sleep, we often feel not only rested, but restored. Some scientists believe that physical and cognitive restoration is actually the reason we sleep. The restorative theory has gained momentum in recent years thanks to a host of compelling animal and human studies. For example, in The Rechtschaffen experiments, performed in the late 1980s, lab animals deprived entirely of sleep lost all immune function and died within weeks. Scientists have also discovered that most restorative functions, including muscle and tissue repair, protein synthesis, and the release of growth hormones, occur during sleep in humans.
Plus, it seems that sleep somehow boosts our immune system. A 2009 Carnegie Mellon University study showed that people who slept seven hours or less each night were approximately three times more likely get sick when exposed to colds than those who have had eight hours or more.
But it’s not just your body that gets rejuvenated while you catch your Zs. When you’re awake, a chemical called adenosine builds up in your brain. By-product of cellular activity, adenosine is thought to tire us out. as it accumulates throughout the day, the more tired we become. During sleep, the body removes adenosine from the brain, allowing us to feel refreshed and alert when the alarm goes off.
Brain plasticity theory
One of the newest and most exciting theories about why we sleep is based on research that suggests that sleep is correlated with changes in the structure and organization of the brain, a phenomenon known as brain plasticity. This theory, sometimes called information consolidation theory, advances the idea that during sleep our brain sorts through the information we have acquired that day, discarding data we don’t need and storing the remain in long-term memories. Several studies support this notion, showing that sleep deprivation negatively impacts our ability to learn and recall information.
Scientists still don’t know exactly how sleep and memories are connected, although many researchers believe that brain waves during different stages of sleep may be correlated with certain types of memories, such as fact-preserving memories. and others who preserve the procedures. The brain plasticity theory may explain why infants and young children, whose brains are still developing, need so much sleep. Infants, for example, spend up to 16 hours a day sleeping, and much of that time is spent in REM sleep, the stage during which most dreams occur.
Whether sleep transforms our brains or conserves precious calories, scientists are unlikely to discover a compelling reason behind sleep. In fact, while these theories may have distinct names and proponents, the reason we sleep is likely a combination of these ideas, or simply all of the above.
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