During the day most people in your neighborhood are usually at work or at school and are not using their internet at home.
So during the day your internet usually is faster because there is less traffic going through the internet lines.
Even when your internet line is not directly shared with other neighbors your internet service does eventually connect to one central internet building where it then connects to one or two major internet lines.
Those lines then lead to other buildings such as data centers and other telephone central offices if your internet is DSL.
If your internet is Cable it then connects to the cable companies other lines and when people jump on the internet at night your internet traffic has to go through a traffic jam and flow through the major internet line along with the other traffic.
So it's normal to see an internet speed slow down at night and then return to normal during the day.