Over 100,000 metric tons of caffeine are consumed around the world every year.
全世界每年会消耗10万多吨咖啡因。
That's equivalent to the weight of 14 Eiffel Towers.
这相当于14个埃菲尔铁塔的重量。
Most of this caffeine is consumed in coffee and tea,
大部分的咖啡因存在于咖啡和茶中，
but it's also ingested in some sodas, chocolate, caffeine pills, and even beverages labeled decaf.
但也存在于碳酸饮料、巧克力、咖啡因片剂，甚至标注了无咖啡因的饮料中。
Caffeine helps us feel alert, focused, happy, and energetic, even if we haven't had enough sleep.
咖啡因帮助我们保持清醒、专注、快乐和充满能量，甚至在我们睡眠不足时也如此。
But it can also raise our blood pressure, and make us feel anxious.
但它也能使血压升高，产生焦虑。
It's the world most widely used drug. So how does it keep us awake?
咖啡因是世界上最广泛使用的药物。那么它是如何使我们保持清醒呢？
Caffeine evolved in plants where it serves a few purposes.
咖啡因从植物中的物质进化而来。
In high doses, as it's found in the leaves and seeds of certain species, it's toxic to insects.
咖啡因在某些植物叶片和种子中含量很高，能够使昆虫中毒。
But when they consume it in lower doses, as it's found in nectar, it can actually help them remember and revisit flowers.
但当昆虫只从花蜜中摄取低剂量的蜂蜜时，有助于昆虫记住并偏好这类花朵。
In the human body, caffeine acts as a stimulant for the central nervous system.
在人体中，咖啡因是中枢神经系统的兴奋剂。
It keeps us awake by blocking one of the body's key sleep-inducing molecules, a substance called adenosine.
通过抑制人体睡眠诱导因子，一种叫腺苷的物质使我们保持清醒。
Your body needs a constant supply of energy, which it gets by breaking down a high-energy molecule called ATP.
身体需要通过分解一种叫ATP的高能分子来得到持续的能量供应。
In the process, it liberates adenosine, ATP's chemical backbone.
在这个过程中将释放ATP的化学主链，腺苷。
Neurons in your brain have receptors perfectly tailored to this molecule.
大脑中的神经元就存在腺苷分子的高特异性受体。
When adenosine docks to these receptors, it activates a cascade of biochemical reactions
当腺苷与这些受体结合，将激活一系列生化反应，
that cause neurons to fire more sluggishly and slow the release of important brain-signaling molecules.
促使神经元活动减弱，从而减缓脑部重要信号分子的释放。
In other words, you get sleepy. Caffeine is what's called an adenosine receptor antagonist.
就是说，你犯困了。咖啡因是腺苷受体拮抗剂。
That means it derails this process of slowing your neurons down by blocking adenosine receptors.
能够通过结合腺苷受体阻断使神经元反应变慢的过程。
Caffeine and adenosine have a similar molecular structure,
咖啡因和腺苷有类似的分子结构，
close enough that caffeine can wedge into the adenosine receptors, but not close enough to activate them.
咖啡因能正好嵌入腺苷受体，但不会激活它们。
To summarize, adenosine inhibits your neurons.
总之，腺苷抑制了神经元。
Caffeine inhibits the inhibitor, so it stimulates you. Caffeine can also boost positive feelings.
咖啡因则抑制了这种抑制剂，所以能刺激你保持清醒。咖啡因也能促进积极的感受。

In some neurons, the adenosine receptors are linked to receptors for another molecule called dopamine.
在某些神经元中，腺苷受体和多巴胺受体连接在一起。
One of dopamine's roles in the brain is to promote feelings of pleasure.
多巴胺可以让人感到快乐。
When adenosine docks in one of these paired receptors,
当腺苷结合自身受体后，
that can make it harder for dopamine to fit in its own spot, interrupting its mood-lifting work.
便能够阻碍多巴胺嵌入受体位点，打断其改善心情的作用。
But when caffeine takes adenosine's place, it doesn't have the same effect, and dopamine can slide in.
当咖啡因代替了腺苷的位置，多巴胺就能正常与受体结合。
There's evidence that caffeine's effects on adenosine and dopamine receptors can have long-term benefits, too,
有证据表明，咖啡因作用于腺苷和多巴胺时也会产生长期的积极作用，
reducing the risk of diseases like Parkinson's, Alzheimer's, and some types of cancer.
比如降低如帕金森、阿尔兹海默症和一些癌症的患病风险。
Caffeine can also ramp up the body's ability to burn fat.
咖啡因也能促进身体燃烧脂肪。
In fact, some sports organizations think that caffeine gives athletes an unfair advantage and have placed limits on its consumption.
实际上，运动组织认为咖啡因会让运动员更加兴奋，因此会限制运动员的咖啡因摄入量。
From 1972 until 2004, Olympic athletes had to stay below a certain blood-caffeine concentration to compete.
从1972到2004年，奥委会规定赛时运动员的血液咖啡因含量必须保持低水平。
Of course, not all of caffeine's effects are so helpful.
当然，咖啡因也不是只有好处。
It might make you feel better and more alert, but it can also raise your heart rate and blood pressure,
它可能让你感觉良好，保持惊醒，同时也会让你心跳加速，血压升高，
cause increased urination or diarrhea, and contribute to insomnia and anxiety.
导致尿量增多和腹泻，产生失眠和焦虑。
Plus, the foods and beverages caffeine is found in have their own impacts on your body that have to be taken into account.
此外，也要考虑食物和饮料中咖啡因对身体的影响。
Your brain can adapt to regular consumption of caffeine.
大脑能接受一定的咖啡因摄入量。
If your adenosine receptors are perpetually clogged, your body will manufacture extra ones.
如果你的腺苷受体不断被占用，身体就会额外产生新的腺苷受体。
That way, even with caffeine around, adenosine can still do its job of signaling the brain to power down.
这就意味着，即使摄入了咖啡因，新生成的腺苷任依然能让大脑反应变慢。
That's why you may find you need to consume more and more caffeine to feel as alert.
这就是为什么你要消耗越来越多的咖啡因来保持清醒。
There are more and more adenosine receptors to block.
因为身体会产生越来越多的腺苷。
It's also why if you suddenly quit caffeine, you may experience an unpleasant withdrawal.
这就是为什么你突然停止摄入咖啡因，愉快的感觉就会消失。
With plenty of receptors and no competition, adenosine can work overtime,
大量腺苷受体没有了竞争者，腺苷就会过度发挥作用，
causing symptoms like headaches, tiredness, and depressed moods.
导致头疼、疲惫以及情绪低落。
But in a few days, the extra adenosine receptors will disappear, your body will readjust,
不过几天后，过量的腺苷受体就会消失，你的身体恢复了，
and you'll feel just as alert as ever, even without an infusion of the world's most popular stimulant.
你会像以前一样清醒，即使没有这个世界上最流行的兴奋剂。