If you don’t know what “sundowners syndrome” is, and have no experience of its horrors, you may not be able to feel the full impact of The Visit. When you first hear about the syndrome, probably when dealing with your aging relatives, it seems as if it must be fictional — a form of dementia that is activated only after sunset.

Night sets in, and elderly sufferers are overcome with symptoms such as angry agitation, paranoia, and hallucinations. They pace, yell, cry, make irrational demands, describe fearful sights only they can see.

In other words, they enact their own pitiable horror show night after night, and writer-director M. Night Shyamalan has made this mysterious syndrome the basis of his amazingly scary new horror film. In it, two children visit their grandparents at their isolated rural home for the first time after a long family estrangement, and are terrorized by the old people’s increasingly bizarre and threatening behavior.