In what kind of contexts should you use 处于/处在?

Question

I just read the article titled 睡眠时间男女不同 日本女性很难入睡, and in the article, I read the following sentence:

据《读卖新闻》下属的网络媒体报道，在几乎所有主要国家中，女性睡眠都比较好，尤其是北欧国家的女性处在容易睡眠的环境中。

Then I learned that 处在 is a synonym of 处于, and the meaning of 处于 is "be".

However, there are also many similar words in Mandarin that mean "be" - 是，为，当, etc... (maybe 处在 includes the meaning of "in" in English but still, 是在 seems to work just like 处在). What is the difference of 处在/处于 from these words and in what kind of situation should these words be preferred?

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UPDATE

Sorry if my question is obscure, but I would like to know the difference between 处于/处在 and other similar words (e.g. 是，为，当), not the difference between 处于 and 处在.

Answer 1

The different between 「处于」and 「处在」is answered well in @dan, and I prefer to explain why Pleco use "be". Actually the mean of the two word can be explained to "be in", and it's used to in a condition or situation.That means it's usually used as 「处于/处在...... 中」，but it is not in this form absolutely. We can also use like this:

「我们必须和一切爱好和平自由的国家和人民团结在一起……使我们的保障人民革命胜利成果和反对内外敌人复辟阴谋的斗争不致处于孤立地位。」 ————毛泽东 《中国人民站起来了》

And if you want more further explanation，you can visit http://www.hydcd.com/zidian/hz/10444.htm

Answer 2

Most of time they are interchangeable.

'处于/处在 ... XXXX' means 'be in the XXXX of ...'. In this context, it's "be in the environment of"

So, 处在容易睡眠的环境中 == 处于容易睡眠的环境中 == be in the environment of "easy-to-sleep" or be in the easy-to-sleep environment.

处于 has more literary flavor than 处在, as 于 is a classical character.

I could be different according to the contexts, but basically, 是=be, 为=for, 当=as.