It seems incredible now but at one time nostalgia used to be considered a psychiatric condition:
"Nostalgia was regarded as a medical disease confined to the Swiss, a view that persisted through most of the 19th century. Symptoms—including bouts of weeping, irregular heartbeat, and anorexia—were attributed variously to demons inhabiting the middle brain, sharp differentiation in atmospheric pressure wreaking havoc in the brain, or the unremitting clanging of cowbells in the Swiss Alps which damaged the eardrum and brain cells." (Sedikides et al., 2008)
Nowadays we know that it's not just the Swiss that 'suffer' from nostalgia, it's most people, to varying degrees. One survey finds that 80% of people feel nostalgic at least once a week.
There's some reason to think nostalgia might be bad for you, as it does have negative components. Nostalgia is often experienced as a loss or longing for what has now gone. But studies suggest that at the same time people experience warm, positive emotions as they remember happy times.
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Source material from PsyBlog