I have Centurylink DSL myself and it does also slow down at night.
I believe the reason the Centurylink DSL or Centurylink internet slows down at night is because of people getting on the internet at night and then it slows the internet down.
More people that are on the internet during the night and so it creates more traffic Jams on the internet lines.
Only so much internet traffic can go through the big dedicated wire and DSLAM at the ISPS servers and office so it creates a traffic jam.
During the day more people go to work and school and so the internet line is less congested.
So there's not too much that can be done unless Centurylink increases the lines capacity which will likely never happen.