What is the reason for disturbing nightmares that make one dread sleep, and are they due to sins and transgressions? If so, is there repentance?

The cause of nightmares cannot be precisely determined, but sins do bring about calamities. The door of repentance is always open, and a Muslim must adhere to acts of obedience, distance themselves from disobedience, and be diligent in reciting the prophetic remembrances (adhkar) and protective supplications, such as reciting Ayat al-Kursi before sleep. They should also have a good opinion of Allah, believing that no harm will befall them as long as they are diligent in reciting the divine protective measures.