Past perfect vs past simple-- had been asleep/ was asleep

Question

It was 5 in the morning, I had been asleep for 5 hours.

It was 5 in the morning, I was asleep for 5 hours.

What's the difference in their meaning?

Answer 1

Five in the morning is an instant in time: you can't be asleep for five hours within that instant, so the second sentence is not correct. If you were asleep for five hours at some point before now, you can use past simple:

I was asleep for five hours

You can be asleep for five hours before five am, and you use past perfect to express this:

It was 5 in the morning, I had been asleep for 5 hours.