Just before going to sleep is the time when I enjoy reading the most.

Hello, everyone.

The following is one of the model sentences I have found in a writing/composition textbook now in use for high school students.

Just before going to sleep is the time when I enjoy reading the most.

I am wondering if it is grammatically correct to take a prepositional phrase like “just before going to sleep” as the subject of a sentence. When I was a student, I was taught from teachers of English that the subject of a sentence should be a noun equivalent.

Or is the sentence above an “inverted” sentence?

Any comment would be appreciated.

Best regards,

.....

曼迪

Regard "Just before going to sleep" to be a period of time, and I think it functions as a noun, not an inverted sentence.
(my shallow opinion )