My cat did a similar thing towards the end. I left a light on and some clothing with my smell in her space. I also tried kitty CBD treats and I noticed she was able to move more freely but it didn’t reduce the howling. Eventually her kidneys began to shut down and she had what the vet assumed was a blood clot to the spine and it was time to say good bye. If kitty is eating and drinking and socializing still I would say try to keep the nighttime environment as friendly and welcoming as you can, it may be they suddenly feel alone and are disoriented by the change.