I don’t know why you sleep at night and not during the day. Perhaps you have a day job or school. And so people sleep and wake up at different times of the day, as required by their activities, the presence of free time and habits. Before the advent of widespread illumination, they slept at night, because it is dark at night and it is difficult to work in the dark. Even in more ancient times, a person could be devoured by wild animals at night, so he had to hide and sleep at night.
The human body and its biorhythms are arranged and tuned in this way. They were formed throughout the entire period of human existence and development. What is the reason for this? During the day, it is easier to work in the light of the sun , at night - it is easier to hide !