I thought nostalgia apps like Timehop were pointless. Then I started using one.

Go to Facebook on your desktop. Go to the most recent photo of yourself. Click the left arrow.

In all likelihood, you're staring at a much younger version of yourself. At your fingertips is an album of years of your life. Mine stretches back to June 2006, starting with awkward prom photos from my final days of high school. My entire adult life — as is the case for most 20-somethings like me — is thoroughly documented online.

You might not make a habit of scrolling through your past in this way. But you probably get tagged in forgotten old photos on Throwback Thursdays. If you're one of the 12 million people who use the app Timehop, you're shown a daily time capsule of all the photos, tweets, and status updates you shared on the same date in previous years. And if you're a Facebook user, you'll soon experience something like this by default: The company is quietly rolling out an "On This Day" feature that will push old photos to your news feed daily.

Our thirst for nostalgia isn't new. Previous generations, after all, made scrapbooks and photo albums. But the sheer amount of data we have about our pasts is unmatched throughout human history. Every few minutes, it's now estimated, we take as many photos as were taken during the entire 19th century — photos that are quickly uploaded to Facebook and Instagram, where they'll sit indefinitely. Yes, the age of social media has made our personal lives more public than ever before, but an equally dramatic shift has been the way it's made our past lives newly visible to ourselves.

A few months ago I started using Timehop — and began waking up every morning to a push notification of my past. I've gotten used to routinely seeing Facebook invitations to parties I went to long ago, and photos of me sandwiched in between close friends and acquaintances I haven't talk to in years. I read tweets I don't remember writing, and see reminders of a family member who died.

All along, I've kept asking myself the same question. Nostalgia is complicated: Sometimes it brings happy memories, but sometimes it hurts. Facebook's new feature has already come under fire for showing users unwelcome photos of dead loved ones. Still, there's something that compels us to look back at the past. What is it? Why do I — and millions of other people — keep opening our Timehops every day?

People think Timehop is silly — until they start using it themselves

Nostalgia was originally thought of as a mental illness. Swiss doctor Johannes Hofer coined the term in 1688, combining a pair of Greek words: nostos (homecoming) and algos (pain). Nostalgia, in other words, was homesickness — an affliction suffered by Swiss students and soldiers who went abroad. Some doctors of the era considered it a "hypochondria of the heart," and thought it could be cured by shaming sufferers until they stopped feeling nostalgia, or by covering them with leeches. If nostalgia got bad enough, some believed, it was possible to die from it.

Sometime after the Civil War, nostalgia stopped being seen as a curable disease. And around the turn of the 20th century, our relationship to the past changed in a fundamental way that's hard for us to grasp today. With the proliferation of the first cheap film cameras, it became possible to amass albums full of photos that served as explicit documents of your personal history. The past was no longer something you had to remember; it became something you saw.

Subsequent technologies — video cameras, then digital cameras, then Facebook and other platforms that store unlimited numbers of photos — have further connected us to our pasts. But Jonathan Wegener, the programmer who created Timehop along with Benny Wong, thinks they don't go far enough.

One of the first photos I saw in Timehop: a memory from four years ago.

"Most web services are all about real time. Nobody reads tweets from more than a day or two ago," Wegener says. "But we feel there's a ton of value in all this old data we're constantly creating." When you happen upon a forgotten photo unexpectedly, he points out, it's much more meaningful than it was on the day you took it. Timehop is his approach to systematically finding long-forgotten photos.

He and Wong got the idea in 2011, though in a slightly different form. Both avid Foursquare users, they created a service that would show you a user with the same check-ins you'd made exactly a year ago. "It's your past overlaid on your present, basically," Wegener says. (They likened it to the ghost you could race in the video game Mario Kart 64, which would drive around the track in an exact reproduction of your previous lap.)

Eventually, it evolved into a daily email that would show your previous check-ins on the same day, and grew to incorporate data from Facebook, Twitter, Instagram, and other networks. In October 2012, it was released as an app. The point of all this wasn't obvious to many people Wegener first described it to, but once they subscribed to it, they understood. "People went from incredible skepticism — like, 'Why do I need another daily email?' — to getting addicted to looking at it every day," he says. The people behind Timehop certainly didn't invent Throwback Thursday, but the phenomenon took hold on Instagram around the same time it launched and helped drive Timehop's popularity: As people shared screenshots hashtagged #TBT, it'd cause others to download the app, leading to a steady climb up the App Store leaderboards with a detectable spike every Thursday.

When I got Timehop, my initial reaction was similar to that of the people Wegener first pitched it to. At first it seemed utterly uninteresting — especially because I don't post to Facebook much, and seldom tweet about my personal life. For the first week or so, I mainly saw tweets linking to articles I only vaguely remember reading, along with a few old Facebook invitations to college parties that I may have attended.

But over time, more meaningful pieces of data from my past began to surface. I started to see photos I'd sporadically uploaded to Facebook over the years: reminders of the year I spent working in India (like the time I saw the Dalai Lama speak), a visit to the house I grew up in, and group photos of my fresh-faced friends from our first year of college. I saw tweets about articles I'd written that meant a lot to me, and others of me saying goodbye to old co-workers at my previous job.

Once I connected my iPhoto account (which includes all the photos I've taken since 2011, and some from earlier), I saw the random, mundane things I sometimes photograph but would never share online: an ordinary weeknight dinner I cooked for my girlfriend, or a Saturday visit to see my grandfather, who died last spring. Sometimes I'd see specific stories play out in slow motion. Over the past few months, for instance, my Timehops featured photos of rough lumber, then milled and cut legs, then a finished piece of furniture — the stool I'd built as part of a woodworking class I took last spring.

I only look at the photos for a moment or so — usually in the morning — but the memories often linger in my head, occasionally popping up throughout the day. And whether or not the photos stir an important, forgotten memory, I've consistently been stunned by how much time has passed since I took them. How the hell was my trip to South Dakota six years ago?

Talking to other Timehop users, it seems that these bittersweet reminders of the passage of time are almost universal — indeed, they're recognized as one of Timehop's defining characteristics. "The other day, it pulled up some photos from a trip I took seven years ago to Seattle," Matt Sevits, a Timehop user from Eugene, Oregon, told me. "I was just blown away by the fact that it'd been seven years. Sometimes it's honestly a little bewildering to see these things constantly popping up, making you realize how quickly time flies."

How nostalgia helps us feel less lonely

"Nostalgia isn't purely hedonic. It's complex," says Tim Wildschut, a psychology professor at the UK's University of Southampton and a nostalgia expert. "A participant in one of my studies once defined it in a way that really rang true with me. He described it as a 'joy tinged with sadness.'"

Most academic study of nostalgia began around 1999, when Wildschut's colleague Constantine Sedikides arrived in the UK and began wondering about the intense feelings of nostalgia he was feeling for his time in the US. (Prior to that, most nostalgia research was by commercial psychologists, who found that people often stuck to the same brands of products they used in childhood — as Wildschut puts it, "Nostalgia sells.")

The researchers began bringing people into their lab and trying to trigger nostalgia, sometimes by showing them certain photos or playing certain songs, but most often just by asking them to write about fond memories. Their stories almost always involved social memories, created with friends or family years ago — and when surveyed afterward, participants reported feeling significantly more loved and connected with others, and had higher levels of self-esteem, compared with a control group.

When asked, the participants said they most often waxed nostalgic when they were otherwise feeling sad or lonely. So the researchers intentionally tried to trigger nostalgia by evoking these feelings (they had the participants read sad news stories, or told them that a test had revealed they had high levels of loneliness). As they suspected, these people then scored higher on tests of nostalgia than control groups.

Facebook's "On This Day" feature, showing a #TBT from my childhood.

Nostalgia, the researchers hypothesized, was a sort of social defense mechanism. "It seems to counteract people's sense of loneliness, or strengthens their sense of belonging," Wildschut says. "We think of it as a clever way of acquiring proximity to important other people in their absence."

For me — and many other Timehop users — this explanation of nostalgia rings true. Last month, for instance, the app showed me a photo of a bike ride I took a few years ago with a close friend who's moved away — the first of many rides we've taken over the years. Though I've never publicly shared a screenshot from Timehop, I sent it to him, and we briefly reminisced over it. Timehop, unlike Facebook and Twitter, is often about intimate socializing between people who share memories, rather than posting things for the world to see.

Though researchers have looked closely for negative consequences of nostalgic reminiscing, they've found few. Cross-cultural studies of people from 18 countries on five continents found they all engage in nostalgia regularly and consider it to be a positive experience. Studies show it can even help people cope with fear of their own mortality. The main exception is people who habitually worry: for them, nostalgia seems to exacerbate the problem and increase anxiety levels. But by and large, the bittersweet emotion of nostalgia seems to have surprisingly positive effects. This may be related to what psychologists call the Pollyanna principle: our tendency, when recalling any given experience, to remember the good much more powerfully than the bad.

In a Guardian article about nostalgia last year, Wildschut described one of the most striking examples of the emotion's therapeutic potential: the many concentration camp prisoners, during the Holocaust, who habitually discussed memories of bountiful family meals and recipes from years earlier. When Wildschut spoke with a survivor about this behavior, the man explained to him exactly why he'd spent so much time talking about food when he was literally starving. He said, "'We used our memories to temporarily alter our perception of the state we were in,'" Wildschut told the Guardian. "'It was not a solution, but the temporary change in perception allowed you to persevere just a bit longer.'"

The problem of painful memories

So nostalgia is a powerful medicine. Does that mean getting a dose of it every morning is a good idea?

Wildschut points out that Timehop's structure is quite different from the way most people have traditionally experienced nostalgia. "When you're engaging in free-form nostalgia, it's up to you which memories you want to think about at any given time," he says. My daily Timehop, on the other hand, provides specific, explicit photos of memories I'm supposed to feel nostalgic about.

I worry that this will make them lose their potency. The shock of the old is what really makes photos resonate with nostalgia: the surprise of seeing something you totally forgot, or never even knew about, rather than dull familiarity of the photos that you framed, put out, and looked at every day. And because Timehop doesn't use any sort of algorithm — it just shows you every single post from the same date in previous years — I wonder whether next year all my photos will have lost a bit of their luster.

When I asked longer-term Timehop users if this became a problem, some confirmed it had — but others disagreed. "It turns out that because I only look at it for a few seconds, I also forget about it a few seconds afterward," Joshua Gans, a daily user, told me. "I wondered whether it'd get tiresome for me after a year, but it hasn't yet."

Anticipating future Timehops has led to another strange phenomenon. The same way Facebook and Instagram users regularly photograph what they're doing simply so they can show the world, some Timehop users are documenting their lives solely to show their future selves. "Sometimes me and my friends will check in somewhere on Foursquare, just for the sake of it coming up years later," Sevits told me. "We're purposely making memories for the purpose of seeing them later. It's sort of weird."

Timehop's Valentine's Day warning.

By far the biggest potential problem with Timehop and Faceook's new "On This Day" feature is that, among the trillions of photos we've posted to social media, there are plenty that people have no interest in reliving. And the cyclical nature of these apps means that unless something changes, any unwanted memories will echo throughout the rest of our lives. For a week in March, Timehop showed me photos of a trip I took with my ex-girlfriend. It didn't particularly bother me (it was a fun trip, and we're on good terms), but it's weird to realize that I will see them for the same week in March every single year, as long as I use Timehop.

Timehop solved this problem on Valentine's Day, with a warning screen that asked "Are you ready for #VALENTIMEHOP? Danger: exes and feels." The warning was thoughtful, and probably necessary for lots of users. Still, there are another 364 days of the year when people can (and do) encounter pictures of their exes and dead family members mixed in with the photos they do want to see.

Facebook, too, has confronted the problem of difficult memories, with mixed results. This past December, when Facebook rolled out its "Year in Review" feature (an algorithmically created slideshow of photos and highlights from each user's year), people reported being blindsided by photos of dead family members, forcing Facebook to issue an apology.

Facebook apparently tried to anticipate this with "On This Day," saying it wouldn't push photos of people you were previously in a relationship with, or people officially declared deceased on Facebook, to your news feed. But lots of people don't actually declare their relatives dead on Facebook or fill out "relationship status" by tagging their significant other — and users have already begun coming forward with stories of seeing dead friends in the middle of their feeds. Facebook may try to implement additional filters to stop this from happening, but the truth is our algorithms probably haven't caught up with the sea of data we've collected.

There's a crucial difference between "On This Day" and Timehop, of course: Users choose to download Timehop, thereby opting into seeing the memories it surfaces. "On This Day," however, pushes nostalgia on people by default — you can decide to have "On This Day" posts appear in your timeline less frequently, but you can't get rid of them altogether. This is especially galling considering that Facebook is a multi-use app: people use it to see what their friends are up to, share interesting articles, or play games. They aren't necessarily expecting to be confronted by painful memories when they open it.

My romantic, Combos-filled Valentine's Day 2014.

But more than any one specific problem, during the months I've used Timehop several people I described it to expressed a vaguer discomfort with the idea. They felt there was something just a little depressing about our memories being mined to produce yet another feed to wake up and look at in the morning. And I get that. Timehop and Facebook's "On This Day" represent yet another core part of being a person — that is, remembering one's past — that we're handing over to algorithms.

Still, I've come around to the idea that it's worth it — and that there's something valuable about the way these apps make use of data we're already amassing, providing some permanence to an otherwise ephemeral stream of photos, check-ins, and tweets. I realize this isn't the case for everyone, and that my reasoning relies on the fact that I've been lucky to experience relatively little real tragedy among my family and close friends.

But I think of it this way. Over the course of the next few decades, you'll meet new friends and slowly fall out of touch with others. You might go through traumatic breakups and meet someone new. Eventually, your grandparents and parents will die, to be replaced by your future children and grandchildren. In much the same way that most of the cells in your body are replaced every 10 years, by the end of your life, nearly every single person you'll know well will have been a complete stranger (or not alive yet) back when you were a child. And you, of course, will have fundamentally changed; though it's impossible to realize in the moment, our interests, goals, and the things we love are changing all the time.

These are hard, scary truths, and they can make us feel lonely and disconnected. But nostalgia is a lifeline to the past. It's a way of linking your current self to all your past ones. It can't slow down the passage of time, but it's a crucial way of connecting to all the people who've filled and defined your life, whether they're still around or not. As North Dakota State nostalgia researcher Clay Routledge told me, "We access memories and experiences from our past that we cherish as a way to remind ourselves, in the present, that things are okay."

Right now, Timehop can't take me back all that far, because I only have a few years' worth of data it can mine. But I can already imagine the ways it could change the way I look back on my life in the future.

I have, for instance, a pair of voicemails my grandfather left me in the few months before he died last year. In the moment, they were pretty ordinary: In one, he wished me happy birthday, and in the other he said hello. But since he's died, they've become extraordinary. When I listen to them and close my eyes, part of me is momentarily tricked into thinking he's still around, leaving voicemails. I'm careful to listen to them rarely, so their power doesn't fade with familiarity.

Timehop doesn't currently include audio. But if it — or another app — someday promised to deliver me my grandfather's voicemails once per year, I'd probably sign up for it.

It might seem strange to intentionally remember someone on such a regimented annual schedule. But this sort of practice actually goes back a very long way. My mom, who's never even heard of Timehop, does pretty much the same thing for each of her parents. Each year, on the anniversary of their deaths, she takes part in Yahrzeit — a Jewish tradition of honoring the dead. The ritual is simple: She lights a candle, says a prayer, and takes a moment to remember.

Joseph Stromberg is a science reporter for Vox.com.

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