Why do we get deja vu?

Deja vu, French for already seen, is the eerie sense that a brand-new situation has happened before. Although it is hard to study because it strikes unpredictably, the leading explanation is that it results from a temporary error in how the brain handles memory and familiarity. One idea is that the recognition system, based in a region called the rhinal cortex, briefly fires a familiarity signal without any actual matching memory, fooling you into feeling you have been here before. Another possibility is a slight timing delay between the two hemispheres of the brain or between perception and memory storage. Deja vu is most common in young adults and tends to decline with age, and it occurs more often in people who are tired or stressed. It is usually harmless, though frequent episodes can be linked to certain types of epilepsy.