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We've all seen those comparison photos. Some president starts out with a full head of dark hair, and a few years later they're totally gray.
我们都看过那些对比照片。开始的时候，一些总统还是满头黑发，几年后，他们的头发就全变成灰色的了。
But stress doesn't really turn your hair white. At least, not the hair already on your head. It can help reveal gray hairs, though, by increasing turnover.
但是压力并不真的能够将你的头发变白。至少，不能让已经生长于头上的头发变白。通过增加流动，压力能够将灰色头发显露出来。
And scientists think it might just speed up the graying process a little, too.
科学家认为压力只是稍微加快了头发变灰的进程。
Each hair gets its color from melanin, a pigment made by specialized cells called melanocytes in hair follicles.
每根头发的颜色来自于黑色素，毛囊中被称为生黑色素细胞的特殊细胞形成的一种色素。
You can have gray or white hairs because the hair itself, which is mostly a colorless protein called keratin is manufactured by different specialized cells in the follicle, which are called keratinocytes.
由于头发本身主要是一种无色的蛋白质（角蛋白），它们是由被称为角质细胞的毛囊中的不同特化细胞所形成，所以你可以有灰色头发也会有白色头发。
And they tend to live longer than the pigment-producing ones.
并且它们似乎比色素产生细胞活得更长。
So hairs can keep growing, shedding, and growing anew, even if those little pigment factories have stopped production.
所以头发一直在生长，脱落再重新生长，即使那些小小的色素工厂已经停止生产。
How long your melanocytes last—and therefore when you "go gray"—is mostly determined by your genetics.
你的生黑色素细胞能持续多久—因此当你的头发变灰时——主要取决于你的基因。
But your head doesn't turn white the second some of those cells die.
但是当那些细胞中的一些死亡时，你的头发不会立刻变成白色。
Since the follicles can't alter hair that's already grown, gray hairs don't start to show until the old, colorful hair has fallen out, which usually happens every few years.
由于毛囊无法改变已经生长出来的头发的颜色，除非有颜色的头发脱落，否则灰发不会开始显现，有色头发的脱落通常每几年才出现。
But, if something makes your hair fall out faster—like, I don't know, being massively stressed out—then it might seem like you're suddenly graying.
但是如果什么事情让你的头发加快脱落—比如，压力过大—那么这会让你的头发看起来像是突然变灰。
Chronic stress is known to trigger telogen effluvium, a condition where the hair follicle jumps ahead to the final stages of its life cycle and the hair attached to it falls out.
已知慢性压力能触发静止期脱发，这种情况是指毛囊抢先一步来到生命周期的最后一个阶段并且依附在上边的头发脱落。
So instead of lasting years, hairs can fall out within a few months of a stressful event.
所以不用持续几年，由于压力，头发在几个月内就会脱落。
The new hair starts growing again right away, so it doesn't cause baldness.
新头发再次立刻生长，所以就不会造成秃头。
And it doesn't affect melanocytes, so for young people, the hair that falls out grows back its normal color.
并且不会影响到生黑色素细胞，所以对年轻人而言，脱落的头发在此生长回原来的正常颜色。
But if you happen to be at an age where the next hairs in line would be gray anyway, it can speed up that turnover.
但是如果你恰好处于一种头发不管怎样都会变灰的年纪，那么这会加快这种转变。
It's not that the stress turned you gray—it just bumped your gray hairs to the front of the line.
并不是压力导致灰发—压力只是让你的灰发来到了前边。
That said, there is some research that suggests stress might make you go gray sooner.
即便如此，一些研究表明压力或许会使你的头发更早变灰。
That's because stress can increase levels of free radicals — atoms or molecules that have an unpaired electron, which makes them unstable and very reactive.
这是因为压力能够增加自由基的水平，自由基是带有不成对电子的原子或分子，这让它们更加不稳定且非常有活性。
They basically try to steal an electron from anything they run into, which messes with key components of cells.
它们会试图从陷入的任何物质中偷取电子，并干扰关键细胞成分。
A 2006 study found that too many free radicals can prematurely age cells because of their toxic effects on DNA, or even kill the cells outright—and that includes melanocytes.
2006年的研究发现由于它们对DNA的毒性作用，过多的自由基能够在使细胞过早成熟，或者甚至杀死全部细胞，包括生黑色素细胞。
Other things known to increase free radicals in your body, like smoking, also happen to be associated with premature graying.
身体中已知的能够增加自由基的其他物质，比如抽烟，也恰好与少白头有关。
But so far, no one has conducted a controlled experiment to conclusively demonstrate the connection between stress, free radicals, and the sudden appearance of gray hairs.
但是目前为止，没人进行过核对实验，最终论证压力、自由基以及头发突然变灰之间的联系。
So stress might make you go gray. Or it might just reveal how gray your hair already is.
所以压力或许让你头发变灰。或者它只是揭露出了你已经有的灰头发。
Thanks for asking, and a special thanks to President of Space Matthew Brant!
感谢提问，特别感谢Space董事长Matthew Brant！
Without our Patreon patrons, we wouldn't be able to make SciShow and answer all your burning questions about the universe.
没有我们的Patreon赞助人，就没有科学秀，我们也无法回答你们关于这个宇宙的问题。
