Can sonnet 18 by Shakespeare be releated to the human body ?

I think this is an excellent question, provided you do not think of it as a literal comparision.

I think he is talking about the difference between body and spirit here. The discussion of whether or not he should use a nature metaphor to speak of her reveals his fundamental problem with trying to use that metaphor: everything in nature either dies or is too harsh. Her body, too, will die, and her corporal beauty is impermanent. So when he talks about summer, he could be talking about a person's body at the height of its attractiveness. When he talks about the buds of May, he may talking about the body of a young, "budding" or developing person. The sun may represent the outward beauty of a person: it is true that sometimes the sun shines too hot, (and beauty stuns or dazzles rather than entices) and sometimes it is "dimmed" (and we can't see the beauty at all. When he says that "every fair from fair sometimes declines" that might also be a reference to our bodies and the potential for us to age and become less fair than once we were. Our bodies are maddeningly mortal: they flash with youth and beauty and then they age until everything we were is eclipsed by the shade of Death (this was especially so in Shakespeare's time).

The spirit, on the other hand, lives. Her essence transcends her body, and he is capable of capturing that essence in lines of poetry; hence, though her body must die, her spirit through him, becomes eternal.