Dogs do bark, its normal for a dog to bark if startled, lonely, bored...
It isn't normal for a dog to bark if all their needs are being met, and they've been trained to ignore various noises etc, if they feel secure and safe, except perhaps in play/excitement...
I find a lot of people still believe they should shut a dog away at night and ignore any crying/barking, which then means the neighbours need to put up with it too. It isn't necessary, thats total bollocks, but some folk really like to cling to the old fashioned methods (or just know no better and see no reason to seek out more up to date information).
Speak to your neighbour, as if they are aware it is an issue and want to resolve it, then they can be pointed in the right direction for help (suggest FB group Dog Training Advice and Support - there are several groups of the same name but the one you want has a HUGE following, 100's of thousands - its free and contains a ton of modern, effective training resources, and its run by trainers/behaviourist/vets).
If they aren't aware/don't care... then keep a diary of the noise and make a complaint.
Local authorities consider prolonged barking/howling (so more than a couple of minutes really) between 11pm and 7 or 8am to be a nuisance - they're less likely to respond if the issue is random single barks or a couple of barks then silence for hours, even if those barks are in the middle of the night.