Why time speeds up as you age: brain scans and physics offer converging answers

Almost everyone over a certain age has felt it: the years accelerating, summers shrinking, the calendar pages turning faster than they used to. Now, two distinct scientific approaches — one rooted in the physics of neural signal decay, another in brain-imaging data from nearly 600 people — are pointing toward the same underlying cause.

In a study published September 30 in the journal Communications Biology, researchers drew on data from the Cambridge Centre for Ageing and Neuroscience (Cam-CAN), a long-term brain-aging project. A total of 577 people, ranging in age from 18 to 88, had previously watched eight minutes of an Alfred Hitchcock television episode — 'Bang! You're Dead' — while functional MRI scanners recorded how their brain activity shifted over time. That particular clip was chosen because prior research showed it produces the most synchronous patterns of brain activity across a wide range of viewers, making it ideal for studying how the brain segments and tracks unfolding events.

The researchers analyzed those recordings using an algorithm called the Greedy State Boundary Search (GSBS), which detects moment-by-moment transitions between stable patterns of brain activity. The finding was consistent across the full age range: older participants' brains shifted to new activity states less frequently, and each state lasted longer than it did for younger viewers.

This suggests that longer [and, therefore, fewer] neural states within the same period may contribute to older adults experiencing time as passing more quickly.— Study authors

The idea connects to a principle that dates back to Aristotle: the more notable events occur in a given period, the longer that period subjectively seems. If an older brain is logging fewer distinct 'events' per minute of clock time, the reasoning goes, that minute will feel shorter in retrospect.

The researchers attributed this pattern to a phenomenon called age-related neural dedifferentiation. In younger people, specific groups of neurons respond selectively to specific categories — face-selective regions, for instance, fire strongly for faces and less so for other objects. With age, that selectivity erodes: the same neuron groups respond more broadly, making it harder for the brain to register where one event ends and another begins.

The idea that this may affect perception and memory in everyday life, including the feeling that subjective time seems to pass faster with age, looks very plausible to me.— Giorgio Vallortigara, neuroscientist at the University of Trento, Italy (not involved in the study)

A separate and earlier line of inquiry reaches a strikingly similar conclusion through physics rather than brain imaging. In a paper published March 18, 2019, in the journal European Review, Adrian Bejan — the J.A. Jones Professor of Mechanical Engineering at Duke University — argued that the culprit is the slowing rate at which the brain acquires and processes mental images as the body ages.

Bejan's argument is physical: as networks of nerves and neurons mature, they grow in size and complexity, lengthening the paths that electrical signals must travel. As those pathways then age and degrade, they add resistance to signal flow. The combined effect is that the brain generates fewer distinct mental images per unit of clock time — and because the mind marks time by the changing of those images rather than by any internal clock, fewer images means time appears to pass faster.

People are often amazed at how much they remember from days that seemed to last forever in their youth. It's not that their experiences were much deeper or more meaningful, it's just that they were being processed in rapid fire.— Adrian Bejan, J.A. Jones Professor of Mechanical Engineering, Duke University

The human mind senses time changing when the perceived images change. The present is different from the past because the mental viewing has changed, not because somebody's clock rings. Days seemed to last longer in your youth because the young mind receives more images during one day than the same mind in old age.— Adrian Bejan, J.A. Jones Professor of Mechanical Engineering, Duke University

Bejan points to eye movement as observable evidence: infants move their eyes far more frequently than adults, reflecting a faster rate of image acquisition and integration.

A third, older framework adds another layer. In a series of 1960s experiments described in his book On the Experience of Time, psychologist Robert Ornstein showed that perception of duration tracks how much new information the mind is processing. Subjects shown more complex or interesting diagrams — for the same actual duration — consistently reported those intervals as longer. Applied to aging, the theory suggests that as people grow more familiar with their surroundings and routines, less new information is being absorbed, and time compresses accordingly. Dr. Christian Yates, a senior lecturer in mathematical biology at the University of Bath, has noted that children, for whom almost everything is novel, must constantly reconfigure their mental models of the world — a cognitively intensive process that stretches perceived time.

There is also a purely proportional account. A year represents roughly 20 percent of a five-year-old's entire life experience so far, but only about 2 percent of a fifty-year-old's — so the same interval of clock time carries a very different psychological weight depending on how much life you have to compare it against. Joanna Szadura, a linguist at Maria Curie-Skłodowska University in Poland who studies how language shapes time perception, told Live Science that while the neural-events hypothesis is well-founded, it must be considered alongside this internal, nonlinear scale by which people measure their own lives.

Neuroscience researchers Muireann Irish and Claire O'Callaghan have also pointed to memory as a mediating factor: the clarity of memories shapes the experience of time, and distinct, well-encoded events give retrospective duration its texture. Fewer novel experiences means fewer distinct memories, which in turn makes a period feel shorter when looked back on.

None of these explanations is mutually exclusive, and researchers are still working to establish their relative contributions. But the Cam-CAN imaging study does offer a practical suggestion buried in its findings.

Learning new things, traveling, and engaging in novel activities may help make time feel more expansive in retrospect. Maybe even more important though, are meaningful social interactions and activities that bring joy, which can also contribute to a fuller sense of time.— Linda Geerligs, researcher at Radboud University, Netherlands, and study co-author

Why it matters — Understanding why time perception accelerates with age — and that novel experiences and social engagement may slow it — has direct implications for how people choose to spend their later years.

Reported by livescience.com, iflscience.com, neurosciencenews.com