For many of us, the worst moment of every day goes something like this. That noise marks our daily return from the mysterious（神秘的） world that we call sleep. We spend a third of our lives asleep, yet（但是） 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, we're in good company, because there's also a lot of scientists don't know about sleep, too. For a long time, it was just something that happened. Everyone assumed（假定） that our brains were hitting the reset（重新安排） 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 asleep, 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（[生理] 神经递质） release deep within our brain, keep our cerebral（脑的） cortex（(植物的)皮层） alert（ 警报） and primed for consciousness. But throughout the day, as our neurons（[解剖] 神经元） break down ATP for energy, the byproduct adenosine builds up and activates（刺激） sleep control neurons（[解剖] 神经元） near the hypothalamus（[解剖] 下丘脑）. A special region in the center of our brain acts as our master（主人） biological（生物的） clock.

  Light-sensitive（敏感的） cells（细胞） in our retinas（[解剖] 视网膜） feed signals deep into that brain region, training neurons to sync up with Earth's 24-hour cycle of day and night. These circadian（昼夜节律的） rhythms（韵律） are the control switch（开关） that tells us when to feel sleepy or awake. When the world goes dark, this master switch tells our pineal gland（[解剖]腺） to increase levels of the hormone（荷尔蒙） melatonin in the bloodstream（血流）, sort of like a chemical lullaby. Feelings of fatigue（疲乏） set in, body temperature lowers slightly, and 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 of 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（黑暗） is increasingly（日益） rare. In the United States, 99% of people live in areas that meet standards for light pollution. And we've got one person to thank for that. Thomas Alva Edison. Edison thought sleep was lazy, unhealthy, or inefficient（效率低的）, even though he took several naps a day.

  But, despite（不管） that hypocrisy（伪善）, his work-more, sleepless view changed our world forever. Illuminated night became a sign of economic progress. That humankind was no longer at the mercy（仁慈） of nature's clock, or so we thought. Artificial（人工的） light can have serious effects on our sleep cycle. When we're exposed（使暴露） 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（开始） of sleep.

  Depression（沮丧）, heart disease, diabetes, and cancer（癌症） have all been linked to chronic（ 慢性的） overexposure to artificial（人工的） light. Until just a couple hundred years ago, it was common for people to fall asleep right after the sun went down, snooze（ 打盹儿） for a little while, wake up around midnight where they would read or study or do other stuff, and 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（坚持） 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, we are massively sleep deprived（剥夺）.

  Most adults average just six and a half hours a night, and teenagers average just five hours on school nights, which is half of what they need. To fight this chronic exhaustion（耗尽枯竭）, we turn to stimulants（刺激物） like caffeine to help our brains ignore the buildup（集结） of a denicine, and then to fight the stimulants, many people rely（依赖） on alcohol, which just sedates（给…服镇静剂） us. It doesn't even help with real, restful（宁静的） sleep. This vicious（恶的） cycle is worth literally（照字面地） billions of dollars a year. It's kind of messed up. I still love you, though, coffee. So what is sleep for?

  In short, we're 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（[遗] 基因） are linked to circadian（昼夜节律的） rhythms（韵律）. Still, there's no consensus（一致） on exactly why our bodies need sleep. We're definitely less active at night, but considering we only burn about 100 fewer calories（卡路里（热量单位）） while sleeping, it's not a very good energy saving（储蓄） strategy overall（全部地）. We definitely do a lot of cellular（细胞的） repair, protein（蛋白质） synthesis（综合）, and general biological（生物的） upkeep while we're in dreamland, but it's not like we don't do that stuff when we're awake, too. Another theory says that our bodies use time of sleep to flush（奔流） out all the neuro-garbage, removing waste products that build up in our neurons and brain cells, and decision-making regions of the brain like the prefrontal cortex, well, they don't get any downtime while we're awake.

  Like, even if you're totally relaxed and you think your mind is clear, your prefrontal cortex is still a prefrontal cortexing. I mean, just try and think about nothing. Go ahead. See? You're thinking about not thinking. Sleep seems to be the only time for this region to power（激励） down and get a break.

  The greatest benefit of sleep may lie in processing（加工） information and consolidating（巩固） memories from throughout our day, letting the brain do all the rewiring that's necessary for thinking better. Sleep-deprived people do worse when learning new tasks, and they're less able to process new information, whereas（然而） 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（发展） in the first place. Snus and animals are easy targets for predators（掠夺者）, so you'd think evolution（演变） 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（点(头)） off without drowning（淹死）, so they only sleep with one-and-a-half of their brain at a time, swimming along using the half of their body that's still awake. And before baby dolphins learn that trick, they take adorable little dolphin naps while their parents keep them afloat（在海上的）. Sleep or similar patterns of rest are seen so universally（普遍地） throughout the animal kingdom（王国） that they must have an ancient origin（起源）, and one clue comes from a tiny ocean-dwelling（居住） worm（虫）. Every night, these worms swarm near the surface of the ocean to feed, and then every day, they sink down deep to avoid light and predators. The worms have special daylight-sensing（感觉到） cells（细胞） on their back, just like the ones in our eyes. When it's dark, those cells trigger（引发） the production of melatonin, just like in our brains.

  As the melatonin builds up, tiny hairs in 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 to the surface to do it all over again. Sleep might have evolved 700 million years ago, the last time that we shared a common ancestor（祖宗） with that tiny worm（虫）. 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 Sleep by David Randall. And also, be sure to check out this half-hour playlist from our friends at The Good Stuff that digs（掘） even deeper（深刻地） into what our brains are doing while we're 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（喷气式飞机） lag（相隔时间）. In fact, just make sure you're subscribed to The Good Stuff in BrainCraft. They are awesome.

  Stay curious（好奇的）.