Under resplendent Douglas Firs, cedars, and Manzanita trees creeps a green shrub with a powerful odor.

It’s called mountain misery, or Chamaebatia foliolosa. It’s native to California. It only grows at elevations between 2,000 and 7,000 feet and only on the western side of the Sierra Nevada Mountains — which is precisely where I went to summer camp as a kid.

All I need is a gentle whiff of mountain misery on the warm summer air and I am transported back to idyllic summer days filled with lake swimming, water balloon fights, and drinking orange soda. The feeling isn’t just one of happy recollection: It’s carried by a strong current of wistful longing for a time in the past. It’s nostalgia.

Nostalgia is powerful: It gets political leaders elected, it’s at the core of billions of dollars of advertising, and was once considered a medical disease. But what is nostalgia and what makes it so powerful? What’s happening in our brains when we’re feeling it and how does it influence our lives?

Researchers are finding that nostalgia can be an enormously beneficial, and potentially life-saving emotion. Clay Routledge, a psychological scientist, has spent much of his career studying what gives our lives meaning and has found that nostalgia plays an important role.

“Nostalgia is one of the self-regulatory tools we use to remind ourselves that we matter,” he tells Inverse.

What is nostalgia?

“The simplest definition of nostalgia is people’s reflection on their cherished memories,” Routledge explains. “That varies from person to person, obviously, but there are some commonalities.”

Routledge says nostalgic memories tend to have the following characteristics:

• A social component: Nostalgic memories typically involve family members, close friends, or romantic partners.
• Personal meaning: The memories might seem trivial to someone else, but because of the personal context, they’re meaningful.
• They occurred fairly far in the past.

In other words, you’re unlikely to feel nostalgic about something that happened yesterday, but you likely have a nostalgic memory or two from a decade ago.

What nostalgia does to the brain

A 2016 study used fMRI imaging to monitor participants’ brain activity when they were exposed to nostalgia-inducing stimuli. When nostalgia was triggered, participants’ brains showed activity in two powerful neural networks: the areas of the brain associated with memory and the brain’s reward system.

The more those systems “worked cooperatively,” the researchers found, the more nostalgia people felt.

A very specific kind of memory is being activated during nostalgia, Routledge says. “Most of the time when you’re nostalgic, it’s autobiographical; you’re accessing what we call ‘self-relevant memories,” he explains.

Studies have shown that smells, like my trusty mountain misery, can be an especially powerful catalyst for nostalgic memories.

If you’re hiking in the Sierras, you’re smelling fresh air, dirt, and too many trees and plants to count. All of those scents are sorted and parsed in the body’s olfactory bulb, which extends from your nose to the base of your brain. The olfactory bulb is directly connected to the amygdala, an area of the brain responsible for processing emotion, and the hippocampus, an area of the brain associated with memory.

Smells from childhood are particularly powerful in this respect because very often our first exposure to scents is in childhood. Our brains form particularly strong connections during this process in case recollection of the smell is important for survival.

Is nostalgia a positive or negative emotion?

In the earliest days of nostalgia study, Routledge says, “it was seen as simply as homesickness.”

This is where nostalgia gets its name: the word was coined by a Swiss medical student, Johannes Hofer, in his 1688 dissertation. In describing the anxieties of soldiers fighting away from home, he combined the Greek word for homecoming, nostos, with the word for pain, algos.

“There’s a tinge of sadness or loss because you know you can’t have that exact thing again.”

But watch someone find an old-school Gameboy or the opening credits of their favorite childhood TV show, and you’ll likely find them smiling. While homesickness is pure ache, nostalgia is a unique mix of ache and pleasure.

“We call an emotionally ambivalent motion, which means it has its complex right as positive features and negative features,” Routledge explains. “There’s a tinge of sadness or loss because you know you can’t have that exact thing again, or someone in the memory is no longer alive. But that sadness comes with, and is often overwhelmed by positivity.”

That positivity is what can make nostalgia such a useful emotion.

Nostalgia as motivation and meaning

Routledge and his nostalgia-studying colleagues have found that nostalgia is actually used to help self-regulate stress signals in the brain. Routledge speculates likely what those soldiers were doing when Hofer observed them and coined the term.

“These soldiers were stressed and homesick and afraid,” he says, “and revisiting cherished memories from home was a way to cope with that stress.”

His own research supports this theory. “We’ve seen that nostalgia seems to bring online these motivational, or self-regulatory processes in the brain that help us down-regulate or mitigate psychological threats,” Routledge says.

Research suggests the social aspect of nostalgia motivates us to engage in “prosocial” behavior. Nostalgia makes us realize the importance of relationships and, in turn, motivates us to connect with friends and pursue romantic relationships.

It’s also strongly associated with optimism and resiliency.

“Nostalgia is a resource that people use to move forward,” Routledge says.

We saw all of these aspects of nostalgia play out during the pandemic. You probably found yourself watching old movies, or listening to old music. When we were in lockdown, we all felt nostalgic for the “before times” when we could see, in person, our family and friends.

Those feelings were triggered by loneliness and a stressful situation, but they were also motivating — propelling us to believe that if we could just make it through, the reward of those in-person visits would be worth the wait.

The next time you find yourself daydreaming about that summer you were 16 or Christmas morning at your grandmother’s house, know that that swirling mix of comfort and longing is doing something important: It’s helping you understand what you want and motivating you to get there.