"Why did you go to bed last night?"

This year I have been teaching English on Saturdays as well as doing a weekday job not related to English teaching. Due to the coronavirus outbreak, the college managers decided that teaching will be done online, but gave us very little time to prepare. I downloaded a conferencing tool and got as far as setting up sessions and inviting students. Today six students joined, four of them for most of the time. Among other things, I reviewed questions with who, what, where, when, how and why (and later whose, which, how much, how many, how many times, how often and how long). One student wrote:

Why did you go to bed last night?

She very quickly changed it to:

Why did you go to bed late last night?

As with many things in English, the ‘wrong’ question is actually more interesting than the ‘right’ one. “Why did you go to bed last night?” is perfectly grammatical and makes sense, but no-one ever asks it because there are basically only two overlapping reasons why any human goes to bed: they are tired (or sick) and/or they have to get up earlier rather than later the next morning. (We might also add boredom, habit or social convention.) 

On the other hand, “Why did you go to bed late last night?” needs a context where the asker knows that the askee did, in fact, go to bed late: either the askee says “I went to bed at (some late time)” or the asker first asks “What time did you go to bed last night?” and got “(Some late time)” as an answer. Asking “Why did you go to bed late last night?” out of such a context is likely to just confuse the askee.