Your internet slowing down at night or in the evening hours is very normal and the internet slow down during the night and evenings is usually caused by an overload or traffic Jam on the internet wires that go through the ISP after it leaves your town.
All internet connections to homes and businesses eventually go to another building where the wires connect and share a big line.
If you're on DSL the internet traffic goes to a telephone central office with a lot of computers and DSLAMS where the internet connection connects up.
That internet traffic is then sent through another bigger wire that leads out to another building and on and on and on until it reaches the server where you're connecting.
The wires or fiber optic cables can only accept so much traffic and data being pushed through so it slows down as a result of more users streaming or downloading videos or viewing websites.
During the evening and night people are streaming videos, movies etc and downloading large files and using the internet more often than they would be during the day so your internet connection is slowing down as a result.