Friend of mine had a small Yorkshire Terrier dog. This dog would not sleep through a night, and was often sick in the night. After a lot of hassle trying to figure it out with vets etc in desperation he switched off his wifi router. Dog slept fine and has ever since provided the router is off.

It might be out-of-human-range noise or maybe it is wifi signal itself, but whatever, animals I know can detect it, so makes sense humans can also. All you need is something in the body with geometry that resonates and there are many many candidates for that. For a small dog exposed to 5 GHz (fast Wifi) 6cm is going to be the approximate order of cavity size for resonating - the skull potentially, the standard 2.4GHz is going to resonate 12.5cm cavities - approaching human skull sizes. There are other factors, but I can see how these frequencies are capable of causing effects on typical animal geometries.

Incidentally just a bit higher than the highest 5G frequencies the signal is absorbed by O2 (oxygen). 5G doesn't run at that because the signal would not travel, but it IS within the capability of the hardware to put out signals that are bang on the resonant frequency of Oxygen... much debate about the potential of that as offensive weapon...