Between the hours of 11pm and 8am wifi and ethernet conections drop significantly

[Snipernant]

For the past few months to almost a year every night between the hours of 11pm and 8am the wifi and ethernet speeds drop. 

We have had technitans out sinc ethis began not for this specific issue but to upgrade to fiberoptic. 
We have replaced our router twice all three have ahad this issue. 
Some nights the slowness begins a 10pm others 1am but it always happens. 

I use both ethernet for my pc and wifi for phones and gadets. most days with my VR heradset its fine as its above 2.5Ghz speeds but one 11pm comes around it alwasys drops below this limit which greatly hinders the playablility of these types of gadets. 

This is a 4 person house, 4 phones, 3 xboxes pc and worklaptop. 3 of these run thorugh ethernet the rest wifi. 

Most of these are turned off at night. with the exemption of the phones and the PC. But even if these were turned on the connection was still grand earlier in the day. 

[Chrisee]

@Snipernant almost impossible to know but if this has affected mutiple routers and apparently 2 different lines (partial fibre and full fibre) it is not going to be easy to find as you have excluded the obvious causes. 

The timing of the fault running overnight would appear to rule out contention as usage falls greatly after midnight and this is actually quite rare in practice in Sky connections . You mention 2.5GHz speeds which might be the speed of the ethernet connection in the home but of course currently the fastest service Sky offer tops out a 900Mb/s (the limit is the WAN feed from the ONT).

Couple of ideas the first is for devices running on WiFi is to download WiFi Sweetspots that measure the connection speed to the router in real time rather than a distant server so showing if the affect is due to the home network or the line. Next is to check your Ethernet connections if they are direct cables to devices they should work or not work but in you have other equipment like powerline adapters or even managed switches they can cause issues.