Question

Grammar check sentence i can't sleep at night. You should

Answer 1

The origin of "at night" to indicate a point of time and the usage of prepositions "in" and"at"

In olden times, when the time expression "at night" was originated, night might have been thought as a point of time in the day because there wasn't any activity going on and people were sleeping that time unlike daytime. It represents the dark hours and the late time of the day. Butmorning, afternoon and eveningrepresent a period of time during the daytime where activities were going on.

In fact, night is a period likemorning. This is the main reason of the question because the preposition in is used for time periods. Then, the question can be easily changed to "why not at morning but in the morning" because it seems like times within the day generally take at (at noon, at 5:00, at dawn, at dinner, at night), except the ones that take inwith the definite article the (in the morning, in the evening, in the afternoon).

Speaking exceptions, one grammar book says the below forat under the title "exceptions: at, on and by":

at night is correct