My internet speed became slow at night

Solution 1:

when your connection seems slow, connect a single computer directly to your modem, run a speed test, ping your DNS for about 30 minutes straight and look for loss and high latency. If speed issues go away, it is something on your LAN.

If speed issues persist then there are many many things that could be wrong. Generally an ISP will want to start with your specific circuit and do end to end testing from the CO to the NID directly. This is a good first step and will rule out any inside wiring in your building. If you still see issues then it can be anything from the lines on the street, the port your connected to, the card the port is on, the shelf the card is on, any internal wiring on the equipment, or even the trunk itself, to mention a few of the more common issues.

Anyone connected to a given DSLAM or CMTS has shared bandwidth through the trunk that feeds the equipment. In lower population areas, these trunks are usually DS3s, limited to 45 mbps for everyone at any given time. If its a higher population area, its probably an OC3 then you have 155 mbps. It doesn't take a lot to fill up that little bandwidth. Ask your ISP to check trunk utilization at the times of day you experience slowness. If its over 80% consistently you can request to have then add another trunk, or move you if they have more than one.

Solution 2:

Most Internet connections use shared bandwidth at some point in the path, for ADSL this will be at the local exchange. The degree of sharing used to be referred to as the contention ratio.

Basically, if your neighbours become more active at night (their sons have all discovered World of Warcraft?) your speed can be affected.

It is possible for ISPs to use "traffic shaping" to throttle back certain types of traffic but I believe this is not the norm - or at least, ISPs rarely admit it. Some ISPs have been known to restrict usage by people who they believe are making unfair and excessive use of their broadband - but I think this mostly applies to a very small percentage of people.