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  • [MUSIC]

  • [MUSIC]

  • For many of us, the worst moment of every day goes something like this

  • [ALARM CLOCK NOISE]

  • That noise marks our daily return from the

  • mysterious world that we call sleep. We spend a third of our lives asleep, yet, other than

  • the odd snapshot of a dream here and there, most of us have no idea what happens after

  • we close our eyes.

  • Luckily were in good company, because there’s also a lot scientists don’t know about sleep, too.

  • For a long time, it was just something that happened, everyone assumed that our brains

  • were hitting the reset button and just turning off for a while. But in the past few decades

  • it’s become clear that sleep might be the single most important behavior that humans

  • and other animals experience.

  • It might seem like we don’t do much while we're sleep, but neuroscience tells a different

  • story. Human sleep patterns are controlled by two competing networks of chemical and electrical signals in the brain.

  • During our waking hours, neurotransmitters released deep within our brain keep our cerebral

  • cortex alert and primed for consciousness. But throughout the day, as our neurons break

  • down ATP for energy, the byproduct adenosine builds up and activates sleep control neurons

  • near the hypothalamus.

  • A special region in the center of our brain acts as our master biological clock.

  • Light sensitive cells in our retinas feed signals deep into that brain region, training neurons

  • to sync up with Earth’s 24 hour cycle of day and night. These circadian rhythms are

  • the control switch that tells us when to feel sleepy or awake.

  • As the world goes dark, this master switch tells our pineal gland to increase levels

  • of the hormone melatonin in the bloodstream, sort of like a chemical lullaby. Feelings

  • of fatigue set in, body temperature lowers slightly, that heat loss is actually why many

  • of us like to fall asleep with our feet sticking out of the covers, true story! Together all

  • this neurochemistry sends one clear message to our bodies: when it’s dark, it’s time

  • to go to bed.

  • Unfortunately, in modern times, darkness is increasingly rare. In the United States, 99

  • percent of people live in areas that meet standards for light pollution, and weve

  • got one person to thank for that: Thomas Alva Edison.

  • Edison thought sleep was lazy, unhealthy, or inefficient, even though he took several

  • naps a day. But despite that hypocrisy his work more/sleep less view changed our world

  • forever. Illuminating the night became a sign of economic progress, and humankind was no

  • longer at the mercy of nature’s clock. Or so we thought.

  • Artificial light can have serious effects on our sleep cycle. When were exposed to

  • bright light at night, our brain doesn’t know better than to think the sun is shining.

  • This can be very confusing, preventing the release of melatonin and the onset of sleep.

  • Depression, heart disease, diabetes, and cancer have all been linked to chronic overexposure

  • to artificial light.

  • Until just a couple hundred years ago, it was common for people to fall asleep right

  • after the sun went down, snooze for a while, wake up around midnight, where they would

  • read or study or do other stuff, then go back to sleep until morning.

  • Modern experiments have suggested that if people are kept away from artificial light,

  • their bodies will return to this pattern of first and second sleep, yet most of us insist

  • on sleeping the whole night through. What’s worse, our circadian rhythms are so tuned

  • to day and night that if we stay up past our usual bedtime, we don’t wake up later, we

  • just tend to sleep less.

  • As a result, were massively sleep-deprived. Most adults average just six and a half hours

  • a night. Teenagers average just five hours on school nights, which is half of what they

  • need. To fight this chronic exhaustion, we turn to stimulants like caffeine to help our

  • brains ignore that buildup of adenosine, and then to fight the stimulants, many people

  • rely on alcohol, which just sedates us, it doesn’t even help with real, restful sleep.

  • This vicious cycle is worth literally billions of dollars a year. It’s kinda messed up.

  • I still love you though, coffee.

  • So what is sleep for? In short, were not really sure, but we know it’s essential

  • to life. Animals deprived of sleep for a long enough time will have seizures, and can literally

  • die from exhaustion, plus a whopping 15% of our genes are linked to circadian rhythms.

  • Still, there’s no consensus on exactly why our bodies need sleep.

  • Were definitely less active at night, but considering we only burn about 100 fewer calories

  • while sleeping, it’s not a very good energy-saving strategy overall. We definitely do a lot of

  • cellular repair, protein synthesis, and general biological upkeep while were in dreamland,

  • but it’s not like we don’t do that stuff when were awake too.

  • Another theory says that our bodies use time asleep to flush out all the neurogarbage,

  • removing waste products that build up in our neurons and brain cells. And, decision-making

  • regions of our brain like the prefrontal cortex, well they don’t get any downtime while were

  • awake, like even if youre totally relaxed and you think your mind is clear, your prefrontal

  • cortex is still prefrontal cortexing. Just try and think about nothing. Go ahead. See?

  • Youre thinking about not thinking. Sleep seems to be the only time for this region

  • to power down and get a break.

  • The greatest benefit of sleep may lie in processing information and consolidating memories from

  • throughout the day, letting the brain do all the rewiring that is necessary for thinkin

  • better. Sleep deprived people do worse when learning new tasks and they're less able to

  • process new information, whereas a good night’s rest appears to make us more creative so we

  • can come up with solutions to new problems that we haven’t seen before.

  • Perhaps the biggest mystery is how sleep evolved in the first place. Snoozinanimals are

  • easy targets for predators, so you’d think evolution would have come up with something

  • better. But it hasn’t. There’s no way to get around the need for sleep.

  • Some animals have come up with interesting ways to deal with the inconvenience of sleeping,

  • though. Dolphins obviously can’t nod off without drowning, so they only sleep with

  • one half of their brain at a time, swimming along using the half of their body that’s

  • still awake. Before baby dolphins learn that trick, they take adorable little dolphin naps

  • while their parents keep them afloat.

  • Sleep or similar patterns of rest are seen so universally throughout the animal kingdom

  • that they must have an ancient origin, and one clue comes from a tiny, ocean-dwelling worm.

  • Every night, these worms swarm near the surface of the ocean to feed, and every day they sink

  • down deep to avoid light and predators. The worms have special daylight-sensing cells

  • on their back, just like the ones in our eyes. When it’s dark, those cells trigger the

  • production of melatonin, just like in our brains. As the melatonin builds up, tiny hairs

  • on their bodies stop beating and the worms begin to sink, just in time for the sun to

  • come up. As the melatonin disappears throughout the day, the hairs begin beating again and

  • they swim back up to the surface to do it all over again.

  • Sleep might have evolved 700 million years ago, the last time we shared a common ancestor

  • with that tiny worm. It's pretty important, so maybe we should all make a little more

  • time for it.

  • If you want to learn more about the science of sleep, one book that really helped me is

  • Dreamland: Adventures in the Strange Science of Sleepby David Randall. And also, be

  • sure to check out this half-hour playlist from our friends at The Good Stuff that digs

  • even deeper into what are brains are doing while were asleep, plus Craig goes to a

  • sleep lab to find out how to get a better night’s rest. Sleeping on the job, Craig,

  • real professional. Oh and over at BrainCraft, Vanessa has a video with some scientific tips

  • on how to beat jet lag. In fact, just make sure youre subscribed

  • to The Good Stuff and BrainCraft, they are awesome. Links to all that down in the description.

  • Stay curious.

[MUSIC]

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B1 中級

なぜ私たちは眠らなければならないのか? (Why Do We Have To Sleep?)

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    Harvey Pan に公開 2021 年 01 月 14 日
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