For many of us, sleep is a precious gift, akin to Coffee, that was gifted to us early on in our evolution. But scientists have long been completely baffled as to just why we sleep, and just what constitutes sleep anyway. A new study attempts to address just why we sleep.

"We don't understand the purpose of sleep, but it must be important because all animals do it," Chiara Cirelli and Giulio Tononi, the study authors say, who describes the search for sleep as like the search for the mythological phoenix.

Some scientists believe that sleep is not important by itself in mammals and birds, and is just a way to impose a quiet and immobile state. Cirelli and Tononi reject this opinion, pointing to the fact that, so far, there has been no evidence of any animal not sleeping.

Even the dolphin, which is often used as an example of an animal that does not sleep because it keeps moving, has developed its own method of sleeping. The dolphin shuts down one half of its brain, swimming with one eye closed, and exhibiting the slow waves characteristic of deep sleep.

"The very fact that dolphins have developed the remarkable specialization . . ., rather than merely getting rid of sleep altogether, should count as evidence that sleep must serve some essential function and cannot be eliminated," Cirelli says.

Cirelli also points to sleep deprivation, and the after-effect of having gone a long time without sleep, as examples of the necessity of it. Sleep deprivation has been shown to kill animals like rats, flies and cockroaches, as well as humans who suffer from genetic insomnia. And when a human rebounds from lack of sleep, they sleep for a long time to recuperate.

Their hypothesis suggests that sleep acts as a way for the brain to regroup after a hard day. Sleep theoretically gives the synapses – which have been escalating in strength during the day – a chance to slow down again, and return to a base level. Given that the brain uses 80% of its energy in order to keep the synaptic activity happening, there is an obvious need for the brain to rest.

They also suggest that sleep allows for the consolidation of new memories, and the trashing of older, random and unimportant memories from the day passed. This theoretically allows for more learning the following day. "While there may still be no consensus on why animals need to sleep, it would seem that searching for a core function of sleep, particularly at the cellular level, is still a worthwhile exercise," Cirelli concludes.

對大多數人來說，睡眠是咖啡的近親；睡眠是寶貴的天賦，是早在人類進化初期就賦予了我們的天賦。但我們究竟為什么需要睡眠？睡眠又究竟是由哪些部分組成的呢？僅僅是這樣的問題，長期以來卻仍然困擾著科學家們。一項新的研究試圖解釋我們為什么睡覺。

這項研究的作者，Chiara Cirelli 和 Giulio Tononi將對睡眠的探究形容為探究神化中的鳳凰，同時他們認為雖然我們還不了解睡眠的目的究竟是什么，但是從所有的動物都睡覺這一點可以看出睡眠一定是非常重要的。

有科學家認為，對哺乳動物與鳥類而言，睡眠本身并不重要，它僅僅是一種強制動物進入靜止狀態的方式。Cirelli 和 Tonon不同意這種觀點，他們指出，目前為止還沒有任何證據表明有什么動物是不睡覺的。

即使是海豚這樣，由于一刻不停地游動而常常被認為不睡覺的動物，其實也早已發展出了其特殊的睡覺方式。在游泳時，海豚會睜一只眼閉一只眼，它們關閉一側的腦部，其腦電波表現出了深度睡眠時的慢波特征。

Cirelli說，海豚發展出了如此不同尋常的特化作用而不是簡單地擺脫睡眠，這一重要事實正可以被看作是證明睡眠具有不可缺少的重要功能的證據。

Cirelli同時將睡眠剝奪及長時間不睡覺的后效作為證明睡眠必要性的例子。已有研究表明，睡眠剝奪會導致諸如老鼠、蒼蠅和蟑螂等動物的死亡，也會引起基因性失眠癥患者的死亡。而一個缺乏睡眠的人會睡很長時間，以此來恢復精神。

他們的假設認為，睡眠是大腦在一天的辛苦工作之后，進行重組的一種方式。睡眠可以使高速緊張的神經突觸得以減速，并回復到基線水平。腦部要用其80%的能量來維持神經沖動的發生，顯然腦部是需要休息的。

同時他們指出，睡眠為鞏固一天中新的記憶，并且拋棄掉一些比較舊的、隨機的、不太重要的記憶提供了可能。從理論上看，這將使新的一天里得以獲得更多的知識。Cirelli總結道：“盡管在動物為什么需要睡覺這個問題上，還沒有統一的共識，但是對睡眠的核心功能的探究，尤其是在其細胞水平上的探索，仍然是一項很有價值的實踐。”