Déjà vu is the strange feeling that the moment happening right now has happened before.
You may know logically that it has not.
The feeling can still be extremely convincing.
Scientists do not have every detail worked out, but déjà vu appears closely connected with the brain systems that produce memory and familiarity.
Familiarity and memory are not exactly the same thing
Normally, the two work together.
You see a familiar person.
Your brain says, “I know this person,” and you can also retrieve information explaining why.
During déjà vu, the familiarity signal may appear without a matching memory.
Your brain says, “I know this.”
Then another system asks, “From where?”
No useful answer arrives.
Can a new place resemble an old one?
Yes.
Sometimes the layout, mood, lighting, sound, or pattern of a new situation may resemble an earlier experience without you consciously noticing the connection.
The brain may register enough similarity to produce familiarity even though you cannot identify the original source.
That can contribute to the eerie feeling.
Is déjà vu a memory glitch?
“Glitch” is a reasonable everyday description, but the brain is not necessarily malfunctioning in a dangerous way.
Memory depends on several systems working together.
A brief mismatch among those systems can produce the experience.
Déjà vu is common and usually harmless.
Does déjà vu mean you dreamed the event before?
Not necessarily.
Because dreams are often forgotten, people sometimes connect déjà vu with an earlier dream.
That may occasionally be true.
But the ordinary experience of déjà vu does not require a prophetic dream.
The brain can generate familiarity without a conscious earlier memory.
Why does it feel so convincing?
Familiarity is usually trustworthy.
When the brain sends a strong “I know this” signal, you are accustomed to believing it.
That is what makes déjà vu so strange.
The feeling is strong even while your reasoning says the situation is new.
Is frequent déjà vu ever a concern?
Occasional déjà vu is common.
Very frequent episodes, especially when paired with confusion, unusual sensations, loss of awareness, or other neurological symptoms, can deserve medical evaluation.
That is different from the brief ordinary experience most people have.
So déjà vu is probably not evidence that you accidentally repeated the timeline.
Your memory system appears to have briefly stamped something “familiar” before checking the paperwork.
A related piece of human behavior appears in Why do we forget dreams so quickly?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why do we forget dreams so quickly?, Why does time seem to go faster as we get older?, and Why can a smell bring back a memory?