"Nostalgia in the brain pairs our memory functions with our reward systems," Dr. Hafeez says. When we encounter a "meaningful" memory, like a place where we had a happy experience, certain neurons fire in the brain that are devoted to emotional processing, according to a study by the University of Surrey and the National Trust in 2017. That tie between memory and emotion is unique to nostalgia.

In a study published in Social cognitive and affective neuroscience in 2016, researchers conducted MRI scans of brains feeling nostalgia, and found out that nostalgic memories depend on two things: "chronological remoteness" (being far in the past) and "emotional and personal significance." When nostalgia was triggered, the brains of the people being scanned showed activity in the memory areas and in the parts that give us "rewards," or positive feelings and sensations. The more those two systems worked in tandem, the more nostalgia people felt.

"These studies document neurological activity in the frontal lobe, the limbic and paralimbic regions," Dr. Hafeez says. Those regions relate to your memory and how you feel pleasure. "Blood flow increases and neurotransmitters are released to the body and activity heightens in these regions producing generally positive responses." This is why nostalgia can make you feel so good.

A study published in 2013 in Personality & Social Psychology Bulletin found that nostalgia doesn't just have to do with the past; it can increase your resilience and your positivity about the future and cause spikes in self-esteem. It's got such good emotional resonance for that you're not just listening to hits from our high school days, you're also refueling your sense of positivity. "The rewards released by nostalgia in the brain can calm a person down, reduce anxiety, and affect your outlook," Dr. Hafeez says.

If you feel more nostalgia in the colder months — candy apples! Halloween! Christmas trees! — it may not just be because of holidays gone by. A study published in Emotion in 2012 found that people feel more nostalgia in colder months, and also experience more "warmth" when they do. When the subjects of the study had nostalgic feelings in cold rooms, they rated their surroundings as warmer and more comfortable.

"Nostalgia can affect your decision-making, which is why it is such an exploited marketing tactic," Dr. Hafeez says. Harkening back to the "good old days" is well-known as an effective strategy for influencing decision-making, and a study in Journal of Consumer Research in 2014 confirmed that when you feel nostalgia, you make bigger purchasing decisions, spending more money in one go. Entering a store that reminds you of your favorite childhood Christmases appears to change how rational you are about money. Result? Lots of shopping.

"Nostalgia can cause us a degree of gloom at times," Dr. Hafeez says. "While this sadness isn’t generally chronic or depressive, it is linked to our ability to accept the inevitability of time has passed." The Neurology Times reported in 2016 that you're more likely to experience nostalgia if you also get high marks on a particular scale designed to measure your capacity for sadness. The Affective Neuroscience Personality Scales track peoples' tendency for play, curiosity, caring, fear, anger, and sadness, and those who show a great tendency for sadness are more likely to experience strong nostalgia more regularly, likely because the emotional centers of the brain are very active.

In 2018, a study published in European Journal of Social Psychology found that in the United States, nostalgia helps people avoid feeling guilt or shame about the problems in the nation's history. The more nostalgic people were, the less likely they were to express guilt about past crimes or problems. "Nostalgia serves as a kind of defence [sic] against bad feelings and as a resource to raise the coherence and moral standing of the group," they said in a press release. And that can have a dark side.

A study in Consumer Communication Reports in 2017 found that nostalgia can actually change the brain's addiction patterns and help people quit smoking. Being exposed to PSAs that evoked nostalgia about life before cigarettes made people in the study more motivated to quit and more negative about smoking in general.

Weirdly, that feeling of longing for times gone by can make it easier to cope with pain. A summary of studies published in Frontiers in Psychology in 2020 found that writing about happy memories can actually increase your pain tolerance — and it even works with people who experience chronic pain.

Studies have found that the more nostalgic you feel, the more you experience something called "self-continuity," where you identify with the same identity over time (rather than looking at pictures of teenage you and going "who IS she?"). One study published in Emotion in 2016 actually found that the more self-continuity you feel from nostalgic memories, the happier you are. "In many ways, nostalgia also helps us remember our roots and has a reflective effect on our life," Dr. Hafeez says.