Your ISP must have installed new "green" power equipment that runs only on solar energy
Can you tell if you lose the modem connection or does the internet not work. Sometime the lights will be different.
Depending on the modem you can try 192.168.100.1 in your browser to see if that is the modem IP. There should be some kind of log. It will show if you are getting errors or if the modem is having trouble talking to the ISP equipment. The ISP can generally see these remotely but they are lazy so if you can find errors you can do part of their job for them.
Generally you can tell if it is the cables from the signal levels....these also should be on a screen you can see in the modem. You can look the recommended levels up but if they get out this range you tend to get connection drops.
Now if you see no messages and the modem itself is not dropping it is more likely something in the ISP cabinet or maybe you have a neighbor that uses his stuff only at night and it is interfering in some way it is not suppose to. Problems that only happen outside the ISP normal business hours are a pain to get fixed.
I had one where I got lots of data loss mostly at night. The ISP said other people near mean also reported this and they happen to get lucky and see it at the very end of their shift one time. They never told me exactly but it seems someone living near by had a unauthorized connection whatever that means.