Hi @2Jays You have my sympathy, especially as I have light sensitivity, but luckily I can wear eye masks, and if I can afford it sometime, I will get black-out blinds.
However, my biggest worry for you is the second concern you raised. If the light now draws attention to your property when previously it was easy to overlook, and you say the rest of the area is dark, then I would have some concerns about your and your neighbours security now.
Does he live alone - sorry, I can't remember whether you said or not - then maybe he was worried that if he had to call an ambulance in the night - and he previously had no outside light - that they would have difficulty manoeuvring him between his house and the ambulance?
If so, could he get the sort of security light that is a motion sensor, but that either stays on until it is reset, or could be set to stay on for the first 10 minutes/half an hour/ hour, after it was activated? I do feel sorry for him if he does have this worry, and he might respond better to the security risk, and motion sensor lights, than knowing it annoys you?
As a sort of aside, I am a pensioner now, who seems to be coming down with more and more illnesses and conditions as I age (expected I know!). I think that as we get older we naturally and unfortunately become less tolerant in general. Hopefully most of us try to keep a lid on our grumpiness (but I must admit that I find shows like "Grumpy old men" and "Grumpy old women" much funnier than I used to do!) Anyway, on top of that I have now got a - still thankfully quite rare -"condition" and one of it's less well known symptoms is that it can actually change one's character, a bit like at least some Dementias. In fact, if you have this medical condition it is quite a lot more likely that you will go on to develop a dementia, particularly Lewy Body Dimentia. So I am having to fight not only my illnesses usual effects, but this extra 'making me grumpy, more sensitive and more disabled one too.
I just wondered if your neighbours illness is making him less tolerant as well - although even if that is the case there is no way that it means you should try and curtail any of your DC activities and fun, except in the normal way that you would to stop them excessively annoying anyone. Good luck OP, you sound nice, and hopefully a friendly chat with your neighbour (would he appreciate something like some home-made soup from you), centering on the security risk, rather than the disruption, might make him think again about his all night light? Otherwise it is probably back to the black-out blinds I'm afraid. 💐