About three years ago Eva Salem got into some trouble with a crocodile. It snapped her hand in its jaws. In a panic, she managed to knock out the crocodile and free herself. Then, she woke up.

"I imagine that's what it's like when you're on heroin. That's what my dreams were like—vivid, crazy and active," she says. Salem, a new mother, had been breast-feeding her daughter for five months before the croc-attack dream, living on four hours of sleep a night. If she did sleep a full night, her dreams boomeranged, becoming so vivid that she felt like she wasn't sleeping at all.

Dreams are amazingly persistent. Miss a few from lack of sleep and the brain keeps score, forcing payback soon after eyelids close. "Nature's soft nurse," as Shakespeare called sleep, isn't so soft after all.

"When someone is sleep deprived we see greater sleep intensity, meaning greater brain activity during sleep; dreaming is definitely increased and likely more vivid," says neurologist Mark Mahowald of the University of Minnesota and director of the Minnesota Regional Sleep Disorders Center in Minneapolis.

The phenomenon is called REM rebound. REM refers to "rapid eye movement," the darting of the eyes under closed lids. In this state we dream the most and our brain activity eerily resembles that of waking life. Yet, at the same time, our muscles go slack and we lie paralyzed—a toe might wiggle, but essentially we can't move, as if our brain is protecting our bodies from acting out the stories we dream.

Sleep is divided into REM and four stages of non-REM; each has a distinct brain wave frequency. Stage one of non-REM is the nodding off period where one is between sleeping and waking; it's sometimes punctuated with a sensation of falling into a hole. In stage two the brain slows with only a few bursts of activity. Then the brain practically shuts off in stages three and four and shifts into slow-wave sleep, where heart and breathing rates drop dramatically.

Only after 70 minutes of non-REM sleep do we experience our first period of REM, and it lasts only five minutes. A total non-REM–REM cycle is 90 minutes; this pattern repeats about five times over the course of a night. As the night progresses, however, non-REM stages shorten and the REM periods grow, giving us a 40-minute dreamscape just before waking.

The only way scientists can study REM deprivation is by torturous sleep deprivation. "We follow the [electroencephalogram] tracing and then when we see [subjects] moving into REM, we wake them up," says psychologist Tore Nielsen, director of the Dream and Nightmare Lab at the Sacré-Coeur Hospital in Montreal. "As soon as you start to rob them of REM, the pressure for them to go back into REM starts to build." Sometimes Nielsen will have to wake them 40 times in one night because they go directly into REM as soon as they are asleep.

Of course there is non-REM rebound as well, but the brain gives priority to the slow-wave sleep and then to REM, suggesting that the states are independent of each other.

In a 2005 study published in Sleep, Nielsen showed that losing 30 minutes of REM one night can lead to a 35 percent REM increase the next night—subjects jumped from 74 minutes of REM to a rebound of 100 minutes.

Nielsen also found that dream intensity increased with REM deprivation. Subjects who were only getting about 25 minutes of REM sleep rated the quality of their dreams between nine and eight on a nine-point scale (one being dull, nine being dynamite).

Of course, REM deprivation, and the subsequent rebound, is common outside the lab. Alcohol and nicotine both repress REM. And blood pressure drugs as well as antidepressants are also well known REM suppressants. (Take away the dreams and, curiously, the depression lifts.) When patients stop the meds, and the vices, they're rewarded with a scary rebound.

But the persistence of REM begs the question: Why is it so insistent? When rats are robbed of REM for four weeks they die (although the cause of death remains unknown). Amazingly, even though we spend about 27 years dreaming over the course of an average life, scientists still can't agree on why it's important.

Psychiatrist Jerry Siegel, head of the Center for Sleep Research at the University of California, Los Angeles, recently proved that REM is nonexistent in some big-brained mammals, such as dolphins and whales. "Dying from lack of REM is totally bogus," Siegel says. "It's never been shown in any species other than a rat."

Some theories suggest that REM helps regulate body temperature and neurotransmitter levels. And there is also evidence that dreaming helps us assimilate memories. Fetuses and babies spend 75 percent of their sleeping time in REM. Then again, platypuses experience more REM than any other animal and researchers wonder why, because, as Minnesota's Mahowald puts it, "Platypuses are stupid. What do they have to consolidate?"

But, given that rats run through dream mazes that precisely match their lab mazes, others feel that there must be some purpose or meaningful information in dreams.

John Antrobus, a retired professor of psychology and sleep research at the City College of New York says that dream content is tied to our anxieties. But he never found the extreme vividness in REM rebound that others assume is there, based on a higher level of brain activity which likely means more action-packed dreams.

"The brain is an interpretive organ, and when regions are less connected as they are in sleep, we get bizarre narratives," he says. "But its purpose? For that we have to ask what is the purpose of thought. We can't answer one without answering the other."

她说：“我的梦真切，疯狂，惊心动魄。我猜吸食毒品后的幻觉大概就会是这样。”Salem当时刚做妈妈，梦见鳄鱼袭击的时候，正是给她女儿哺乳的第五个月，当时她每晚只能睡上4个钟头。就算难得有个整觉可睡，又会做这些梦，梦境像是真的一样，弄得她觉得自己像是根本不是处在睡眠中。

令人惊讶的是，梦会一再重复出现。缺乏睡眠的人，大脑不得不持续工作，所以等人一合上眼，它就开始报复。所以，梦，莎士比亚笔下“大自然的温情的保姆”，并不是那么的温情。

“当人睡眠不足，睡眠强度就会增加，也就是说睡眠期间大脑会更加活跃，所以梦就会更多，也更生动。”明尼苏达大学神经学专家、明尼苏达睡眠失调中心负责人Mark Mahowald解释道。

这种现象叫做快速眼动（REM）反弹。REM是rapid eye movement（快速眼部活动）的缩写，指眼球在眼皮下飞快运动。在这个阶段我们最容易做梦，大脑活动也和清醒时惊人的相似。然而与此同时，我们肌肉放松，全身麻痹般躺着——也许能偶尔抽动脚趾，但总的来说动弹不得，这就像是大脑为防止我们身体像梦到的一样动作而采取的保护机制。

睡眠过程可分为快速眼动阶段和四个非快速眼动阶段，每个阶段的脑波频率都不一样。非快速眼动的第一阶段是瞌睡阶段，人处于半睡半醒之间，有时会伴有类似落入洞中的感觉。在第二阶段，大脑迟缓下来，只有少量活动。在随后的第三第四阶段，大脑才真正休息，进入慢波睡眠，此时心跳和呼吸频率都大大降低。

只有经过70分钟非快速眼动睡眠，我们才开始第一次快速眼动，这一阶段只持续5分钟。一整个非快速眼动-快速眼动周期时长90分钟，一晚大约重复5次。然而越到后来非快速眼动过程越短，快速眼动过程越长，睡醒前最后一次快速眼动可以带来40分钟的梦境。

科学家目前研究快速眼动缺乏的现象只有通过人为中断志愿者睡眠的方法。蒙特利尔圣心医院梦境实验室的负责人、心理学家Tore Nielsen说：“我们追踪脑电图，当观察到被试者进入快速眼动，就叫醒他们。当快速眼动被人为打断时，被试者会强迫自己重新回到这个状态。”有时Nielsen一晚上要叫醒他们多达40次，因为他们一入睡就直接进入快速眼动状态。

当然也有非快速眼动的反弹，但这只是由于大脑倾向于先进入慢波睡眠，再进入快速眼动，这表明这些过程是彼此独立的。

在2005年发表于《睡眠杂志》的论文中，Nielsen指出，一个晚上缺少30分钟快速眼动，会导致次日晚快速眼动增加35%——被试者快速眼动过程由74分钟跃增至100分钟。

Nielsen还发现，梦的强度随着快速眼动的缺乏而增长。给只经历25分钟快速眼动的被试者做梦的质量评分，在满分9分的评价系统，可达到8～9分，其中1分为完全不活跃，9分为非常活跃。

在生活中，快速眼动的缺乏和随之发生的反弹非常常见。酒精和尼古丁都会抑制快速眼动。控制血压的药物和抗抑郁药物也会遏制快速眼动。当停用药物或是戒烟戒酒时，人们就会面对可怕的反弹。

我们研究快速眼动的反复出现，同时却回避了一个问题：为什么它会那么顽固的存在？老鼠被剥夺快速眼动过程持续四周就会死亡（其中机理仍然未知）。尽管平均人的一生按这规律做梦的时间累计长达27年，科学家对于梦何以如此重要仍各执一词。

洛杉矶加州大学睡眠研究中心的主任、精神病学家Jerry Siegel最近证实，某些脑部较大的哺乳动物如海豚和鲸鱼没有快速眼动现象。“缺乏快速眼动会导致死亡的论调是站不住脚的，”Siegel说，“没有在任何除了老鼠以外的物种身上观察到这种现象。”

一些理论认为，快速眼动有助于调节人体体温和神经传导素水平。有证据显示做梦能帮助巩固记忆。胎儿和婴儿有75%的睡眠时间处于快速眼动。动物中鸭嘴兽快速眼动时间最长，对此科学家却表示不解，因为，用明尼苏达的Mahowald的话说，“鸭嘴兽很笨，它们有什么好巩固的？”

也有人考虑到老鼠睡梦质量和实验室表现高度匹配，认为梦一定具有一定的含义，能传达某种信息。

纽约市大学心理与睡眠研究所的退休教授John Antrobus认为，梦的内容和我们的焦虑有关。然而，理论上大脑活动加剧，可能造成梦的内容更加丰富，但他并没有观察到这种有人认为在快速眼动反弹阶段会出现的梦境异常生动的现象。

他说：“大脑是一个解释性的器官，当我们睡着，大脑各部分联系不再那么紧密，我们就会梦见千奇百怪的故事。但要问这有什么目的性，我们就得先问问自己思想的目的是什么。回答不了这个问题，也就回答不了前一个问题。”