What Happens During Déjà Vu
During déjà vu, your brain recognizes something as familiar even though it is actually new. Your brain processes familiarity separately from actual memories. When these two systems get confused, you feel like you have seen or experienced something before, even though you know logically that you have not. The experience feels very real and automatic.
The Memory Malfunction Theory
The most accepted explanation is that déjà vu occurs when your brain's memory system has a glitch. Your memory is split into different types: semantic memory (facts and general knowledge) and episodic memory (personal experiences). Sometimes a current situation triggers your semantic memory of similar things, making your brain feel like you experienced this exact moment before. This mismatch between what feels familiar and what you actually remember creates the déjà vu sensation.
Why It Happens More Often
Déjà vu is more common when you are stressed, tired, or distracted because your brain is not paying full attention. Similar situations, like walking into a room arranged like another room you know, can trigger the feeling. Travel to new places and encountering new people also increase the chances of experiencing déjà vu because you encounter more novel situations that might partially resemble past experiences.
Related Experiences
Déjà vu is part of a family of similar experiences. Jamais vu is the opposite, where something familiar suddenly feels completely new and strange. Presque vu is when something is on the tip of your tongue. These all point to how your brain sometimes struggles to correctly process memory and familiarity information.
Is It Normal?
Yes, déjà vu is completely normal and not a sign of any medical problem. It is not dangerous and does not mean anything is wrong with your memory or brain. Occasional déjà vu is a common experience that most people will have multiple times throughout their lives. 
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