When you sleep, your brain restarts and cleans itself.

当你睡觉时，你的大脑会重新启动并自我清理。

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When you sleep you get the following stages.

睡眠有以下几个阶段。

耶鲁公开课 - 心理学导论课程节选

When you're really tired sleep is what you want to do like when you're really hungry you want to eat.

你累了，就会想睡觉,正如你饿了想吃东西一样。

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When you're in dreamless sleep, you're not imagining or experiencing anything.

当在无梦的睡眠中时，你没有想象或者体验任何事。

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Even the noise you make when you sleep, can't swim across the river so deep.

NPR: The Arcade Fire, Dark and Energetic

Getting comfortable when you sleep is important because a lack of sleep can cause joint inflammation and lowers your pain threshold, experts say.

WSJ: Find the Perfect Sleep Position

"We aren't sure why activity primes your body for sleep so well, but it's likely a combination of factors, including lowering your core body temperature, increasing the sleep-promoting hormone melatonin and supporting a biological need to restore energy levels and repair cells and tissues when you sleep, " Cardinal says.

CNN: How to fix health problems with exercise