In fact, they are like a dream. The dream seems quite real, just as in waking life the physical world is quite real. Nor is it of use to say there is nothing real to be afraid of to someone who is terrified of spiders. The fear is quite real. The feeling of threat is real.
Thoughts then arise that compound and reinforce the feeling.
We know only too well how in a crisis the mind manufactures thoughts that always seek to establish the current mental state. Even powerful emotions do not last. If something has really upset me, and we know how that goes, I go over the grievance in my mind, re-visiting it again and again. But just try to maintain the level of anger. A point will come when it begins to subside.