Brown is a Puritan, who is (or was) very faithful to his faith. During the period, witch trials and meetings were frequent, so it was always on his mind. Nightmares usually happen when worries or fears are on someone's mind. Since he has a dream of being apart of a witch-meeting and while those who mean the most are as well, he feels like he sinned. Nightmares can have adverse effects if the person allows it to control them. After the nightmare, Brown begins to act outlandishly. When "Deacon Gookin was at domestic worship, and the holy words of his prayer were heard through the open window [Brown says] ‘What God doth the wizard pray to?'" then, later, Brown finds Goody Cloyse teaching to a young girl, so Brown snatches the girl away from Cloyse.
The supernatural scenes of the devil, the time when Brown sees Goody Cloyse, and how the nightmare effects Brown are reasons why Goodman Brown had a dream in the forest of a witches'