What is the significance of seeing a disturbing nightmare after performing Qiyam al-Layl for the first time?

A disturbing dream is a frightening and saddening experience from Satan. A Muslim should act according to the prophetic guidance in this situation, for the Prophet, peace and blessings be upon him, explained that dreams are of three types: the righteous dream, which is good tidings from Allah; a dream that causes sadness, which is from Satan; and a dream that is merely what a person thinks about himself.

So, if one of you sees something he dislikes, let him stand up and pray, and not relate it to people. Let him spit to his left three times, and seek refuge with Allah from the evil of Satan and from its evil, for then it will not harm him.

The questioner is advised to memorize and adhere to the remembrance (adhkar) before sleep.