Why we dream continues to elude us. Scientists have proven we need to dream. When robbed of their dreams, rats die within four weeks.

We also know that at seven months a fetus is dreaming, its muscles and eye movements giving the tell-tale signs of REM （or rapid eye movement） sleep and non-REM sleep. But what happens before seven months? When do our dreams begin?

Research published in Chaos, a journal of The American Institute of Physics, provides the first attempt at an answer.

Mathematicians analyzed the brainwaves of a fetal sheep in utero, at 15-weeks. The brain signals at that stage are quite complex, set against noise that is difficult to dampen. But using sophisticated mathematics, scientists discerned a pattern of cortical activation and deactivation, cycling every five to ten minutes - this, the scientists note, is a crude precursor to the longer cycles of REM and non-REM sleep.

We can only guess at the content-do sleep dream of electric androids? But the study shows that dreamlike sleep develops before rapid eye movements. And the discovery may give researchers new insight into the purpose of sleep and dreams.

我们何时开始做梦

我们为什么做梦的问题一直难以解释。科学家们已经证明，我们需要做梦。不让老鼠做梦，它们在4周内就会死亡。

我们还知道7个月大小的胎儿也做梦，胎儿的肌肉和眼动能清清楚楚地反映出快速眼动睡眠（REM）和非快速眼动睡眠（non-REM）信号。但是，7个月之前是什么样子呢？我们是何时开始做梦的？

发表在美国物理学会《混沌》（Chaos）杂志上的研究试图第一次给出这些问题的答案。

数学家分析了子宫内15周的绵羊胎儿的脑波。对比那些难于减弱的噪音信号，这个时期的脑波信号已经非常复杂。但是通过使用复杂的数学计算，科学家们发现了 一种皮质激活和去活的模式，这种活动每隔5到10分钟循环一次。科学家们注意到，这种脑波活动模式就是周期更长的快速眼动睡眠（REM）和非快速眼动睡眠 （non-REM）的粗略前身。

我们只能仅仅猜测梦的内容--睡梦中会出现电子机器人么？但是该项研究表明，梦一般的睡眠在快速眼动之前就有了。而且该研究发现可能会对睡眠和梦的目的给出新的见解。