Please tell the meaning of the second "ït" in this sentence. thank you.
It(first one) is fairly clear that sleeping period must have some function, and because there is so much of it(second one) the function would seem to e important.

Question

Please tell the meaning of  the second "ït" in this sentence. thank you.
It(first one) is fairly clear that sleeping period must have some function, and because there is so much of it(second one) the function would seem to e important.

Answer

Diego is correct. The second 'it' does refer to 'sleeping'.

The first 'it' is sometimes called an 'empty' subject.
We use it as a meaningless subject with expressions that refer to time, weather, temperature, distances, or just the current situation.
It's ten o'clock.
It's Monday again.
It rained for three days.
It's thirty degrees.
It's ten miles to the nearest petrol station.
It's terrible - everybody's got colds, and the central heating isn't working.
Wasn't it lovely there!

Answer

1.  This sentence is not correct.  "Sleeping period" is wrong.  To begin with, we do not use such a phrase as "sleeping period".  Secondly, even if we did, it would need an article:  "a sleeping period", "the sleeping period"; or it would have to be plural: "sleeping periods".  What does "sleeping period" mean?  
  
2.  The syntax is awkward.  What is the sentence trying to say?  Is it trying to say that sleep must serve a useful function?  If so, the way to write it is "We (or babies, or snakes, or an appropriate subject) spend such a big proportion of our lives sleeping.  It must serve an important function."

3.  If you want to keep as much of the existing structure as possible, it will have to be, "It is clear that sleep serves a useful function.  There is so much of it that it must be an important function."   This would still be very clumsy and awkward English, though grammatically correct.

4.  The correct way to ask your question is "Please tell me the meaning of the second 'it' in this sentence." The verb "tell" needs a direct object "me" here.  Alternatively, you could ask, "Can you tell the meaning of the second 'it' in this sentence?", but then here "tell" would take on the different meaning of "decipher" and not "communicate".  Do you want it to mean "communicate (to me)" or "(are you able to) decipher"?

This is not to pick faults but to learn together how to write English.

Answer

The second one it refers to sleeping !   -- "... And because  there is so much of it (sleeping) the function would seem to be important. "  That's what I think,  someone else can help you if they see this post