Is that 8 MB (Mega BYTES)/s, or 8 Mb (Mega BITS)/s?
If it's advertised at 8 MB (Mega BYTES)/s speed, Then that *Can't* be a "T1" line... "T1" is only 1.544 Mb (Mega BITS)/s over Dual-Copper pair Phone line...
8 MB/s sounds more like DOCSIS Cable (or possibly VDSL?) service, 8 Mb/s could Possibly be ADSL... (?)
In any case, if you are experiencing Slowdowns ONLY at Night ("Prime-Time") hours, and GOOD speeds in Daytime ("Off-Peak") Hours otherwise, then there's NOTHING that YOU can do, other than to keep complaining to your ISP, and asking them to check into it DURING the "Prime-Time" Hours (WHEN the problem is Actually Happening)...
Be Persistent, calm, but FIRM that they need to do something to correct this issue... Contact Others in your area, and see if they also are experiencing this same issue... If so, Get together and Complain as a GROUP... There's Power in Numbers...
If You're Unsure, "RTFM"... If You're SURE, "RTFM" Anyway.