"during which" = "when"??

Some English grammar books say that you can use <during which> instead of <when> in a relative adverb clause. Is it always true?



Well, I'm not sure.



Certainly, <during which > and <when> are both time-related expressions.

However, while <when> in a relative adverb clause modifies time noun such as “the period”, “the age”, <during which> is not always the case.



you can say,

I watched a movie <during which> I fell asleep.



But you can’t say,

I watched a movie <when>I fell asleep.



Am I correct?



If so, what do you think about the following sentences?

Are both OK?

Which one do you prefer?



1. The period <during which> people first learned to melt iron is known as the Iron Age.

2. The period <when> people first learned to melt iron is known as the Iron Age.



Please help me.

Thanks!