"Fall asleep" VS "Drop off"

Question

Scenario one)

Please imagine someone has been awake because of many thoughts and worries that has been thinking about the whole last night. (These thoughts did not allow him to sleep.)

The person gets up in the morning and goes to work and his colleague finds him drowsy and asks why are you so sleepy? The person can say:

a. I couldn’t fall asleep last night.
b. I couldn’t drop off last night.

I was wondering if the both sentences mean the same here. (I think they mean the same, but I guess some people from different countries / regions may find them not interchangeable here, while "drop off" has some more common meanings to them.)

Scenario 2)

Now, let's imagine he and his wife are preparing to sleep, but after a while, the wife notices her husband keeps tossing and turning in the manner that is obvious the man is still up! What should the wife ask her spouse to discover why her husband cannot sleep?

a. Why don't you sleep?
b. Why don't you fall asleep?
c. Why don't you drop off?

Answer 1

Scenario one)

a. I couldn’t fall asleep last night.

This is correct. There are other ways to say it, but this is the standard way.

b. I couldn’t drop off last night.

Usually "drop off," "nod off," or even "droop off" is more like what this person would be doing during their work the next morning: slowly starting to fall asleep while trying to remain awake (because they are bored or drowsy). That being said, "drop off" can just be a slang term for "fall asleep," so b. is technically fine.

Scenario two)

a. Why don't you sleep?

This is less common.

b. Why don't you fall asleep?

This is the most correct.

c. Why don't you drop off?

Again, if both people often use the phrase "drop off" to mean falling asleep, then this would work. Where I am from, this phrasing is not common and could be a little more derisive.

Also, "why don't you ..." is a suggestion, like "how about you fall asleep?" If the wife were asking a question out of concern, she might say something like "why can't you fall asleep?" or similarly, "why can't you sleep?" Using "why don't you" does imply that she is frustrated. "Why don't you fall asleep?" is closer to "please fall asleep now" than it is to "what's wrong, honey?"