It might seem strange that two identical periods of time could feel vastly different, but in fact, your sense of time is incredibly flexible. Fear, excitement, and awe can all make time seem to slow down, while repetition or being "in the zone" can make it seem to speed up. The return trip effect seems to be another instance of your time perception being all loosey-goosey — it's just that researchers can't agree on what exactly that return trip does to your brain to make it feel so short.

There are a few different hypotheses, and we'll explain them one by one.

Our hunch was that the return trip feels shorter because you know it better than you did on the way there, and a study published in 2015 bears this out. Researchers from Kyoto University had participants watch videos of a person walking two of three routes: one from what we'll call point A to point B, another from point B back to point A, and a third on a completely different route — point C to point D. They consulted a map of the route as they watched their video. Only those who watched a round trip — point A to B and back to A again — felt the second trip was shorter. That was despite the fact that during the videos, they estimated that the exact same amount of time was passing. That suggests that the return-trip effect is a matter of your brain revising the past, rather than experiencing time move faster in the moment.

A 2016 study published in the journal Hippocampus sheds a little more light on what the brain might be doing to shrink the return trip. Researchers asked first-year college students to sketch a map of their campus, complete with travel time estimations between various points. They found that the more familiar a student was with the area, the larger the spaces they sketched and the shorter the travel times they estimated.

This might seem like a home run for the familiarity theory — but science is rarely that neat and tidy. A 2011 study published in Psychonomic Bulletin & Review contradicted those familiarity studies by switching the routes up. Researchers from the Netherlands and the U.S. had participants estimate the travel time in three real-life scenarios: a bus trip, a bike ride, and a lab test where they watched videos of someone else traveling. In all three experiments, people said the return trip felt shorter — even when the return trip was on a completely different but equidistant route.

These researchers thought this might be because you underestimate the time it'll take to get to the destination — leaving you disappointed when it takes longer than you thought and likely to overestimate the time it'll take to get back. When that overestimation of the return trip proves to be overblown, it feels delightfully shorter. Indeed, the participants who most reported feeling like the initial trip took longer than expected experienced the greatest return-trip effect.

Researchers for a 2016 study published in the Journal of Experimental Psychology had another hunch: Maybe the initial trip feels longer because you don't know what to expect when you get to your destination. To find out, they brought students into one room of a lab, took them to another room one floor down to do a letter-search puzzle, then took them back to the first room. Half of the students were told where they were going and what they'd be doing, and the other half were kept in the dark. Those who didn't know what to expect when they left the room judged the initial trip to be longer than the trip back.

All of these hypotheses — familiarity, overestimation, and anticipation — match up with the idea that time seems to move more slowly the harder your brain is working. For example, in 1975, Robert Ornstein showed participants either a circle or an irregular polygon and asked them to remember it. Even though the same amount of time passed before they were asked to recall the shape, those who were assigned the circle felt less time had passed than those who were assigned the polygon.

When you're seeing new landmarks, wondering what the destination will be like, or repeating "are we there yet?" to your parents' dismay, your brain is doing more work than when you're seeing the same landmarks or thinking about what's in the fridge at home. Regardless of the exact scientific explanation for why the way back feels shorter, it's obviously your brain playing with your perception of time. In the end, maybe it's best to just be happy you're finally home.

Get stories like this one in your inbox or your headphones: Sign up for our daily email and subscribe to the Curiosity Daily podcast.

For more about your brain's relationship with time, check out "Your Brain Is a Time Machine: The Neuroscience and Physics of Time" by Dean Buonomano. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.