Jellyfish exhibit signs of a sleep state, which could mean that sleep predates the evolutionary development of central nervous systems.

　　水母有睡眠状态的迹象，这可能意味着睡眠先于中枢神经系统的进化发展。

　　Humans are hardly alone in their need for sleep—as any cat owner knows. But what about other, very different animals?

　　绝非只有人类需要睡眠，任何一个猫奴都心知肚明。不过，对于那些与人类截然不同的其他物种呢?

　　“In thinking about this…we wanted to use an evolutionary argument to figure out what is the simplest animal to sleep.” Ravi Nath, a graduate student of biology and bioengineering at Caltech.

　　加州理工学院生物学与生物工程系的研究生Ravi Nath表示：“在思考这个问题(其他物种需睡眠否)时，我们想要从进化的观点出发，找到需要睡眠的最原始生物。”

　　“We decided to approach it from a conservation point of view, how conserved the sleep state is. And looked back into the animal tree and specifically looked at jellyfish.” Claire Bedbrook, also a biology and bioengineering graduate student at Caltech.

　　“我们决定从行为恒定传承的观点处理这个问题，究竟睡眠有多古老。并回溯动物种系进化图谱，还专门研究了水母。”同为加州理工学院生物学与生物工程专业的研究生Claire Bedbrook如是说到。

　　Jellyfish have no central nervous system. “They do have neurons—their nervous system is a sort of loose set of neurons that control the animal’s behavior. There’s no control center.”

　　水母没有中枢神经系统。“它们的确拥有神经元，但是它们拥有的是一种控制动物行为的弥散式神经系统。无控制中枢。”

　　Nath, Bedbrook and the rest of their team worked with a species known as the upside-down jellyfish. “Unlike jellyfish that we’re probably most used to seeing in the wild, these jellyfish actually rest their bell on the seafloor, or in our case the tank bottom, and they pulse in place.” These habits made this jellyfish easy to observe.

　　Nath, Bedbrook和团队中的其他成员一起研究了一种以上下颠倒而闻名的水母。“与我们在野外常见的水母不同，这种水母其实会把伞膜搭在海底(在我们研究中是搭在水槽底部)，并就地涌动。”该水母的这些习性令其易于观察。

　　The researchers recorded the animals’ behavior during the day and at night, to see if these jellyfish met the scientific criteria for sleep. “Those three behavioral criteria are, first, that the animal goes through a period of quiescence where they are less active. The second is that during this quiescent state, the animals are not as responsive to environmental stimulus. And the third is that this quiescent state is actually necessary for the animal’s well-being and survival.”

　　研究人员日夜记录此水母的行为，进而观察它们是否达到了科学睡眠的标准。“科学睡眠的三种行为标准如下，第一，动物会经历一段活动减少的静止期。第二，静止状态期间，动物对环境刺激的反应性降低。第三，这种静止状态实际上对动物的健康和生存来讲是很有必要的。

　　The observations revealed that these jellyfish do in fact sleep. That’s a big deal because for scientists “there was this assumption in the community that you needed to have a certain level of complexity”—namely a central nervous system—“to actually sleep. And I think this study really challenges that assumption.” The finding is in the journal Current Biology. [Ravi D. Nath, Claire N. Bedbrook et al., The Jellyfish Cassiopea Exhibits a Sleep-like State]

　　观察结果显示，该水母的确会睡眠。这可是个大发现，因为对科学家来讲“科学界中存在这样一种假设，即必需要进化到拥有一定程度的复杂性(如中枢神经系统)才能够睡眠。我认为该研究确确实实地质疑了上述假设(的真实性)。”该研究发现发表于杂志《当代生物学》-Ravi D. Nath与Claire N. Bedbrook等人，《仙女水母展现出类睡眠状态》

　　So what does this study tell us about why animals sleep, and how sleep evolved? Bedbrook says that although more work needs to be done, they have some ideas. “One hypothesis that we really like is that any animal that has neurons will have a sleep state, and that sleep might be an intrinsic property of neurons.” Nath agrees. “So neurons have a huge metabolic cost, and so maybe this period of sleep is a period of consolidated energy conservation that has evolved with neurons essentially.”

　　那么，关于动物因何睡眠与睡眠如何进化，该研究是如何回答这两个问题的呢?edbrook表示，虽然仍需进一步研究，但是他们已经有一些想法了。“其中有一种假设我们都很欣赏，即任何拥有神经元的动物均会出现睡眠状态，同时睡眠或许是神经元固有的性质。” Nath持一致观点。“由于神经元需要消耗大量新陈代谢的能量，因此或许这段睡眠时期，本质上是与神经元共同进化的一段统一的能量节约时期。”

　　So nerves might be keeping you up at night—but having nerves is what’s probably putting you to sleep.

　　因此，虽然神经紧张也许令你夜晚无法入眠，但正是因为拥有神经，你才能够睡觉呀。