Five or six times a day, every day, for 48 years, chronobiologist Robert Sothern has counted off 60 seconds in his head and then compared the results against a clock.

As part of a lifelong experiment on circadian rhythms, Sothern, now 69, is trying to confirm or reject a widely held belief: Many people feel that time flies by more quickly as they age.

So far, Sothern's results are inconclusive. It's true that lately, according to his measurements — and his gut — time seems to be speeding up as he nears his 70s. "I'm tending now to overestimate the minute more than I used to," he tells me. But then again, he had detected a similar pattern — more overestimates — in the 1990s, only to have his estimates fall in the 2000s. "Time estimation isn't a perfect science," he says.

This matches what other researchers have found too. There's very little scientific evidence to suggest our perception of time changes as we age. And yet, we consistently report that the past felt longer — that time is flying by faster as we age. What's going on?

There's considerable evidence that time doesn't speed up as we age

There are a few different ways to study how we perceive time. Scientists can look at time estimation, or our ability to estimate how long a minute passes, compared with a clock. (This is what Sothern is doing.) They can also look at time awareness, or the broad feeling that time is moving quickly or slowly. Finally there's time perspective, the sense of a past, present, and future as constructed by our memories.

What researchers have found out is that while time estimation and time awareness don't change much as we age, time perspective does. In other words: Our memories create the illusion time is accelerating.

Our ability to estimate how long a minute takes seems to stay constant over time. In 2005, researchers at the University of Central Florida and Westfield State College conducted a version of Sothern's time estimation test. One by one, they brought 100 participants ages 20 to 69 into a dark lab room fitted with only a chair, table, and stopwatch. Like Sothern, the participants were asked to guess the length of various time intervals.

The results? There weren't many differences between the old and the young. "[C]hronological age showed no systematic influence on the perception of these brief intervals of time up," the authors wrote. (That said, the researchers did find that males overestimate time while females underestimate it, perhaps due to having slightly different circadian clocks and therefore slightly different metabolic rates.)

But stopwatches can only tell us so much. After all, we don't just care if our perception of how long it takes coffee to brew changes over time. We want to know if events over the course of months and years actually do fly by.

So to get a sense of a person's time awareness, researchers have to rely on surveys. In 2005, researchers in Munich asked 499 people ages 14 to 94 broad questions about how aware they are of the passage of time, such as, "How fast did the previous week (month, year, decade) pass for you?" and, "How fast does time usually pass for you?"

Here, too, age seemed not to matter. Older people didn't seem to be aware of time passing any faster than younger people. The only question that yielded a statistically significant difference was, "How fast did the last decade pass?" Even there, the reported differences were tiny, and the effect appeared to plateau around age 50.

Reviewing several similar studies and conducting two of their own in two separate nations (and with a total of 1865 subjects), psychologists William Friedman and Steve Janssen found scant evidence that the subjective experience of time speeds up with age. They write in their 2009 paper, "We can concluded that when adults report on their general impressions of the speed of time, age differences are very small."

But then why do so many people feel like time is going faster?

The research finds one twist, however. When people reflect back on their own life, they feel like their early years went by very slowly and their later years go by more quickly. This could be the source of the belief that time goes more quickly as they age.

But how could this be if our time estimation and time awareness were not changing at all? One possibility is that participants were simply biased by the (incorrect) conventional wisdom — they reported their later years as flying by more quickly because that's what everyday lore says should happen.

Another possibility is that this is an illusion, that our minds trick us into thinking our later years are flying by more quickly. "Most people feel that time is currently passing faster for them than it did in the past," Janssen writes me in an email. "They have forgotten how they experienced the passage of time when they were younger."

In their 2009 paper, Friedman and Janssen outlined several hypotheses for why our memories might trick us into believing time is accelerating:

1) Novel experiences are remembered more clearly — and as we age, there are fewer of them

The assumption behind this hypothesis, Friedman and Janssen write, is that "humans gauge the magnitude of past intervals of time ... according to how many events can be recalled from that period." We use significant events as signposts to gauge the passage of time. The fewer events, the faster time seems to go by.

Childhood is full of big, memorable moments like learning to ride a bike or making first friends. By contrast, adult life becomes ordinary and mechanized, and ambles along by. William James, a 19th-century psychologist, phrased the idea in these bleak, beautiful terms:

Each passing year converts some of this experience into automatic routine which we hardly notice at all, the days and weeks smooth themselves out in recollection, and the years grow hollow and collapse.

Each new minute represents a smaller fraction of our lives. One day as a 10 year old represents about .027 percent of the kid's life. A day for a 60 year old? .0045 percent. The kid's life is just... bigger.

Also, our ability to recall events declines with age. If we can't remember a time, it didn't happen.

2) Time flies when we're busy or distracted — and adults are busier than children

Another possibility is that being busy can somehow trick our memory into feeling like time is going by faster. "Tasks that demand considerable attentional resources are perceived as briefer than tasks that are undemanding," Friedman and Janssen explain. As we age, these tasks — career-related tasks, raising children, etc. — increase.

It's also possible that as adults, we feel like we never have enough time to do things — which our brain then interprets as time speeding up. "[F]inding that there is insufficient time to get things done may be reinterpreted as the feeling that time is passing quickly," they write. Deadlines always come sooner than we'd like.

3) Very memorable events are farther than they appear

No doubt you've seen a Facebook post saying something like, "Want to feel old? The baby on the cover of Nirvana's Nevermind is now 80 years old." This sort of thing can fill us with dread, making us think that time has flown by more quickly than it actually has.

Psychologists have long understood the phenomenon called "forward telescoping" — i.e., our tendency to underestimate how long ago very memorable events occurred. "Because we know that memories fade over time, we use the clarity of a memory as a guide to its recency," science writer Claudia Hammond writes in her book Time Warped. "So if a memory seems unclear we assumed it happened longer ago." But very clear memories are assumed to be more recent.

If clear memories form our personal timelines, then everything we remember clearly will seem more recent than it actually was. Realizing that it isn't makes us feel like time has passed us by.

Want time to feel like it's slowing down? Make the most of it.

If our memories can trick us into thinking time is moving quickly, then maybe there are ways to trick our brains into thinking that time is slowing down — such as committing to breaking routines and learning new things. You're more likely to remember learning how to skydive than watching another hour of mindless television. Take a breath and say, "Batman Begins really did debut 10 years ago, and I'm okay with that."

Sothern says when he looks at the data he's collected, he can appreciate all the time he's already been afforded. "I'm so aware of the seasons," he says. "By having a rhythm, it proves you are alive."