为什么有人越喝咖啡越犯困？

上班前喝杯咖啡唤醒一天是越来越多打工人的选择。人们将咖啡作为一种晨间能量饮料，对抗疲劳保持清醒。但是，也有一部分人喝咖啡后反而感觉更困了，这是为什么呢？

The reason we get tired throughout the day is because a neurochemical called adenosine builds up and passes through receptors, which causes nerve cells to become less active and blood vessels in your brain to dilate. In short, your body gets the message that it’s time to rest. But since caffeine resembles adenosine, it binds to these receptors instead—blocking the actual adenosine from passing through and preventing your body from slowing down.
我们之所以会感到疲劳是因为一种叫做腺苷的神经化学物质在体内积累并和腺苷受体结合，这会导致神经细胞变得不活跃，大脑血管扩张。简而言之，你的身体就会接收到休息的信号。但由于咖啡因与腺苷相似，它会与这些受体结合，从而阻止腺苷与受体结合，让人保持清醒。

Though caffeine can keep you awake for a while in this way, it doesn’t cause all that adenosine to vanish forever. “Just because our brain is no longer processing the adenosine doesn’t mean it stops producing it,” Michelle Worley, a registered nurse, told PureWow. “When the caffeine inevitably wears off, you’re left with an adenosine buildup which makes you feel even more tired.”
虽然咖啡因可以让你清醒一段时间，但它不会让腺苷永远消失。注册护士米歇尔·沃利告诉PureWow网站：“我们的大脑仅仅是不再对腺苷做出反应，但并没有停止生产腺苷。当咖啡因不可避免地消失时，体内积累的腺苷会让你感觉更累。”

It’s not unlike getting a burst of energy after eating something sugary, only to crash soon after. And speaking of sugar, if your go-to coffee concoction involves lots of it, that could contribute to post-coffee fatigue, too. There’s also the possibility that caffeine’s diuretic quality can cause dehydration, which can make you feel tired (though some studies have suggested that caffeine doesn’t dehydrate us nearly as much as we’ve been led to believe).
这和摄入一些高糖食物后迅速补充能量，但很快就会消耗掉是一个道理。说到糖，如果你喝的咖啡中含有大量的糖，也可能会导致喝咖啡后更加疲劳。还有一种可能性是咖啡因的利尿性会导致脱水，这会让你感到疲劳（尽管一些研究表明咖啡因不会像我们所认为的那样导致脱水）。

Lastly, you may have built up such a high tolerance for caffeine that a cup of joe just doesn’t perk you up like it used to.
最后一个原因是，你可能已经对咖啡因有了很高的耐受力，一杯咖啡已经不能像过去那样让你振作起来了。