When we were children, each day lasted a lifetime. It seemed that we waited years for holidays and birthdays to come, and summer vacation stretched on into a warm, sunny eternity.

But when we got older, something changed.

We blink, and days have gone by. We nap, and months pass. We sleep, and years pass. Holidays, birthdays, and vacations come and go, passing us by like a truck in the night and barreling on into the past.

As we age, time seems to speed up, making these experiences go by all-too-quickly, making “Where did the time go?” into a too-familiar complaint—one often filled with regret. We blame this on our busy adult lives and grown-up worries and responsibilities, but according to recent research, something else is to blame for this accelerated time travel.

The answer lies in the way human memory works.

According to recent research, our brains verifiably perceive time differently as we age. A 2005 study by psychologists Marc Wittmann and Sandra Lenoff at the Ludwig Maximilian University of Munich found that subjects over 40 perceived time very differently from younger people—for this older group, time seemed to go by much faster.

When the over-40 group was asked to recount their lives, they recalled time moving slowly during their childhoods, but that it also perceptibly sped up through their teens and young adult years.

So, now that we know that your grandparents aren’t just spouting empty platitudes when they warn you that the years will pass you by more quickly than you think. But the question remains: why does this happen?

To find the answer, we look to the way our brains encode information.

One factor is found in the way we see time: we perceive a period of time as the proportion of time we’ve already experienced. This means that since one year is a quarter of a four-year-old’s life, waiting a quarter of a lifetime for that next birthday party is intolerable. On the other end of the scale, one year is only a tiny fraction of an 80-year-old’s life, and will feel like that tiny fraction.

But the biggest factor here is that human beings are wired to pay attention to new experiences. Researchers think this might stem from the fact that the new nearly always represented either a threat or a benefit to early humans. Those who were quick to pay full attention to that poisonous snake or that tree laden with nourishing fruit were the ones to survive, passing on those genes to their offspring.

In fact, our brains are so attuned to novelty that we can use this property to enhance learning. An article in Scientific American, appropriately titled, “Learning by Surprise,” talks about recent cognitive research in this field, showing that we store information more easily, and retain it longer, in a novel environment.

The article goes on to explain that a part of your brain called the hippocampus compares incoming information to the information that is already stored in the brain. If this incoming information differs, the hippocampus sends out a signal that strengthens attention and memory.

Our brains also tend to deeply encode new experiences rather than familiar ones that it has already committed to memory. Think of your very first day at your job. You probably remember the smell of the building, what you wore, and the work you did. But try to revisit the days between then and now, and you’ll have trouble. That’s because your brain was paying the most attention when everything was new.

And so because our judgment of time is based on the number of new memories we make during a certain period, an experience that is full of new memories will feel as if it took longer than an experience full of familiarity.

This is the biological reason why time seems to pass more quickly as we age—for better or for worse, our brains simply glaze over the old.

When we’re young, the world is new, and is filled with experiences we have yet to explore. But as time goes on, we experience more and more, and so everyday life becomes wholly familiar. We encode fewer new memories, and so our brains simply skim over all of the things it “already knows.”

And thus, time flies.

This is, in fact, good news! Your grandparents probably never thought about why time seemed to fly by for them. They only know that it did. But now that you know the mechanisms behind how you perceive time, you can do something about it.

Think about it. By purposefully pursuing new experiences, you can slow down time for yourself.

The speedy nature of time can be a huge source of regret. Too often, we look back at the latter parts of our life—particularly life after 40—and see little more than a blur. If you want to avoid this, it might just be time to break out of your normal routine and embrace the weird, the new, and the fantastic.

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