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.)
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?