We don't have synonyms for sundowning yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are landowning and undawning.
(n) :
(medicine, psychology, psychiatry) A state of agitation, confusion, or delirium, especially in patients suffering from Alzheimer's disease, which begins daily at nightfall and which is alleviated by daylight.
(n) :
(medicine, psychology, psychiatry) A state of agitation, confusion, or delirium, especially in patients suffering from Alzheimer's disease, which begins daily at nightfall and which is alleviated by daylight.