Dreams can contain impossible events.
You are back in school.
Your dog is driving.
The building has no stairs.
During the dream, none of this seems especially suspicious.
Sensory brain systems stay active
During dreaming, especially in REM sleep, brain areas involved in visual imagery and emotion can be highly active.
That helps create scenes, faces, motion, fear, excitement, and other vivid experiences.
The brain is generating its own internal world.
Reality-checking is different
Some parts of the prefrontal cortex involved in critical evaluation and logical control are less active during typical REM sleep.
That can reduce the amount of questioning you do.
The dream story does not have to survive the standards you would apply while awake.
Emotion can make the experience convincing
Dreams often contain strong emotion.
Fear.
Embarrassment.
Joy.
Confusion.
Emotion makes experiences feel important and immediate.
That intensity can help the dream feel real even when the plot makes very little sense.
Why do dreams fade after waking?
Once awake, the brain shifts into a different state.
New sensory information floods in.
Dream memories can be fragile.
That helps explain why we forget dreams so quickly.
Why do nightmares feel especially real?
Fear systems can be highly active during nightmares.
The emotional response is genuine even though the threat is not.
Your heart may race.
You may sweat.
You can wake with the fear still active for several moments.
Are dreams only random brain activity?
That is too simple.
Dreams clearly use memories, concerns, emotions, and recent experiences.
Scientists still debate exactly what dreaming does and how much specific dream content matters.
The safe answer is that dreaming comes from an active sleeping brain, not from a single simple mechanism.
The dream feels convincing partly because the part of your brain asking hard questions has reduced office hours.
And that’s the short explanation.
Related explanations: Why do we forget dreams so quickly?, Why do we have nightmares?, and Why do we sleep?