The reason why the cat keeps meowing at the door at night:
1. If your cat keeps meowing at night, it may be because it is hungry. You can give it some food and then observe its condition.
2. If your cat keeps meowing at night, it may be bored. In this case, you can find some toys that your cat likes to play with so that it has something to do.
3. If your cat keeps meowing at night, it may be in heat. Cats will meow a lot after they go into heat. At this time, you can breed it, which is the fastest solution. If you don't want your cat to have kittens, it is best to have it spayed or neutered.
4. It's also possible that the cat litter needs to be changed, and the owner's help is required.
5. If your cat is arching its back and meowing continuously, it may be suffering from severe abdominal pain due to illness. You should take your cat to the vet immediately.
6. It's possible that the cat wants to go outside. It may perceive this as a restriction on its freedom and is meowing to signal to its owner to let it out.
7. It could be a way for the cat to assert its dominance. Check around your home for any free-roaming or stray cats. These cats leave their scent when they appear near your door, causing the cat to meow incessantly. 
How to prevent your cat from meowing constantly at night:
1. Cats are more active at night. Owners can reset their cat's biological clock by playing with it more during nap time and at night to tire it out, which can gradually change the cat's biological clock.
2. If your cat meows in the middle of the night, prepare plenty of food for it before nightfall to prevent it from getting hungry. This will prevent your cat from waking you up in the middle of the night.
3. A dirty litter box might be the reason your cat keeps meowing at night. Cats like clean litter boxes, so owners should clean them before going to bed so the cat has a clean place to relieve itself.
4. If a cat has visual problems or impaired cognitive function, a nightlight can be used at night to reduce fear, which can help reduce nighttime meowing.
