If you're using DSL internet it's very common for the internet connection to slow down during the night because during the evening and night a lot more people are home and get onto the internet.
When all those people get onto the internet and start browsing the internet, watching videos, streaming movies etc it creates a traffic jam on the phone lines and that is what causes the DSL internet to slow down some during the night.
There's not much that can be done about it unless the telephone company increases the capacity of it's lines but that's very unlikely that they will do that.