Why Your Decades Are Disappearing

"The present moment always will have been." — Søren Kierkegaard

Childhood feels longer than adulthood. Everyone notices this eventually. The summer you were nine seems to have lasted two years. The summer you were thirty-four seems to have lasted four days. Your twenties happened, technically, but try to recover them and you'll find a thin slurry of impressions where you'd expect a decade.

This is not nostalgia. It is not a trick of perspective. It is the actual mechanical output of how your brain stores time.

Your hippocampus, the seahorse-shaped structure in your temporal lobe that handles memory encoding, uses novelty as its primary filter. New things get filed. Repeated things get compressed. If today looks like yesterday, your brain doesn't open a new folder. It just adds today to the existing one. You lived 365 days but your brain archived something closer to 40 of them.

This essay is about why that happens, what three mechanisms accelerate it, and what you can do about it. The philosophers were noticing this two thousand years before neuroscience could image it. Seneca had a word for the time you live but don't actually inhabit. Bergson built an entire metaphysics around it. They were describing, in their own vocabularies, the same thing the fMRI studies are now confirming.

The decades are not gone. They were never filed in the first place.

How memory actually gets filed

The hippocampus does not store memories permanently. It acts as an indexing system, tagging experiences for transfer to long-term storage in the neocortex. And it is extraordinarily selective about what gets tagged.

Its primary criterion is novelty.

When something new enters your experience, a new environment, an unfamiliar face, an unexpected outcome, the hippocampus fires. It flags the moment as significant. It opens a new folder. When something familiar enters your experience, the same commute, the same desk, the same sequence of Tuesday, the hippocampus largely stays quiet. It has already seen this. There is nothing to file that isn't already filed. The experience happens. The body moves through it. The brain does not record it as a separate event.

This is not a flaw. It is an elegant efficiency. The brain handles enormous amounts of sensory input every waking second and cannot encode everything. Novelty is the heuristic it uses to separate signal from noise. New things might matter. The same thing you've done a thousand times probably doesn't need another entry.

The cost of this efficiency becomes apparent when you look back at a year of identical days and find almost nothing there.

The technical term is temporal compression. The brain collapses repeated similar episodes into a single generalized memory rather than storing each instance separately. You don't remember 300 individual Tuesday commutes. You remember "the commute," an averaged composite of what commutes were like. The specific Tuesday where it rained and you were running late might have its own entry, because something changed. The other 299 Tuesdays were filed together, as one.

This is why people consistently underestimate how much time has passed during routine periods and overestimate how much has passed during eventful ones. A two-week vacation full of new experiences feels longer in memory than three months of unchanged routine, even though the clock ran identically for both. Psychologists call this the holiday paradox. Vacations feel short while you're on them, because everything is novel and time accelerates in the present, but feel long in retrospect, because so many distinct memories were encoded that the period seems to have lasted forever. Routine periods do the opposite. They drag in the present but compress catastrophically in memory.

Your thirties almost certainly had fewer firsts than your childhood. The filing cabinet was filling more slowly. That is the entire reason your thirties feel shorter.

There are three mechanisms doing this erasure, and they compound on each other.

The first is the one we just covered. Novelty collapse. Jeffrey Zacks at Washington University has spent much of his career studying how the brain segments continuous experience into discrete episodes. His event segmentation theory, confirmed with fMRI, revealed something precise. The hippocampus and medial prefrontal cortex fire in discrete bursts at moments the brain identifies as transitions. Each burst corresponds to what Zacks calls an event boundary, a perceptual moment where the brain effectively says "new chapter." These boundaries become the retrieval points later. When you try to remember a period of your life, you're navigating between these markers. More markers mean more to navigate. More to navigate means the period feels fuller, longer, more richly inhabited. Participants in Zacks's studies who experienced more event boundaries during a given period remembered significantly more of it afterward. Participants whose experience was relatively flat showed compressed, poorly differentiated memory of the same elapsed time.

The second mechanism is sleep. Every night during slow-wave sleep, the hippocampus replays the day's encoded episodes in compressed form, transferring them from short-term hippocampal storage to long-term neocortical storage. This process, called memory consolidation, is not optional. It is when experience actually becomes memory in any durable sense. What the hippocampus encoded during the day is not yet a long-term memory. It is more like RAM, volatile and subject to loss. The transfer to the neocortex during sleep is the equivalent of saving to disk. Skip the save, and the file is gone when the machine restarts. Matthew Walker's research at UC Berkeley has documented this in granular detail. Sleeping less than six hours significantly reduces consolidation efficiency. Chronic sleep debt means a substantial fraction of daily experience never completes the transfer. The events were lived. They were even initially encoded. They never made it to long-term storage. They are simply gone.

The third mechanism is the most insidious because it happens in real time, while the experience is still unfolding. Novel stimuli trigger the ventral tegmental area, a dopamine-producing region in the midbrain, to release dopamine directly into the hippocampus. This dopamine signal acts as a gate. It tells the hippocampus this moment is worth encoding. Attention and emotional salience work the same way. The brain marks moments it was fully present for. The opposite of this is the default mode network, a set of brain regions that activate when attention is not directed at the external world. The default mode network is the neural substrate of mind-wandering. Replaying the past, rehearsing the future, scrolling without registering, sitting through a conversation while actually composing a reply to an email that happened three hours ago. When the default mode network takes over, the dopamine gating signal to the hippocampus quiets down. The hippocampus stops actively tagging the present. Experiences pass through without being marked for encoding. You were at the dinner table. Your default mode network was somewhere else entirely. Your hippocampus, receiving no dopamine signal to flag the moment, did not open a folder. The evening will not be distinctly retrievable. It will collapse into the generalized impression of "dinners we had that year."

The dinner happened. The memory did not.

What the philosophers saw

Seneca's essay On the Shortness of Life, written roughly two thousand years ago, opens with an observation that lands differently once you understand the neuroscience. "It is not that we have a short time to live, but that we waste a great deal of it."

Seneca was not making a productivity argument. He was making a perceptual one. His observation was that most people, at the end of their lives, are shocked by how little they seem to have lived. Not because the years weren't there. Because they were not inhabited. They were passed through without presence, without attention, without the kind of engagement that creates something retrospectively recoverable.

He distinguished between tempus, clock time, the mechanical passage of hours, and what he called vita, lived life, time that is actually experienced and possessed. His argument was that most people accumulate enormous tempus while living very little vita.

The neuroscience of event segmentation and hippocampal encoding is, in essence, the biological mechanism behind exactly this distinction. The hours of tempus run identically regardless of what you do with them. The vita, the encoded retrievable subjectively felt life, is built only in moments of novelty, presence, and transition. You can pass through decades of tempus and build almost no vita at all.

Seneca called it squandering. The hippocampus just doesn't open a folder.

Eighteen hundred years later, the French philosopher Henri Bergson spent his career grappling with the difference between scientific measurable time and what he called durée, duration, the felt texture of time as it is actually experienced from the inside.

Bergson's central insight was that these are not the same thing and should not be confused. Clock time is uniform, divisible, external. Lived duration is elastic. It stretches when experience is rich and varied, and compresses when experience is thin and repetitive. Memory, for Bergson, was not a recording of clock time. It was the accumulation of durée. And durée varied enormously depending on the quality of attention and the novelty of experience. He was describing, in the language of philosophy, what Zacks would later confirm with fMRI. Time as it is subjectively lived is not a function of hours elapsed but of experiential texture encoded. A rich varied hour contains more durée than a flat routine week. And in memory, the rich hour feels longer, because it gave the brain more to file.

Bergson also argued that modern industrial life, with its routines, its standardization, its reduction of experience to repeatable units, posed a specific threat to durée. That it would compress lived time even as it expanded clock time. That people would live longer and experience less.

He was writing in 1889. He was describing 2025 with uncomfortable precision.

How to live so the hippocampus has something to work with

Every intervention follows from the same mechanism. Give the hippocampus something to file. Create event boundaries. Trigger the novelty signal. Keep attention present long enough for encoding to begin. Protect the sleep that completes the transfer.

The minimum viable novelty intervention is route variation. Literally taking a different path. New routes expose the brain to new spatial information, triggering the hippocampus's deep involvement in spatial navigation and episodic memory. The research on this is not subtle. The hippocampus evolved substantially around spatial navigation. New environments reliably activate it.

Beyond that, new food matters more than it sounds like it should. Not as a foodie practice but as a cognitive one. Taste and smell are among the strongest memory-encoding triggers the brain has. A meal at a restaurant you've never been to has a better chance of being filed distinctly than a Tuesday dinner you've had 200 times. New people matter for a different reason. Conversations with unfamiliar people require genuine attention. You cannot autopilot through them, and that attention is exactly the condition for hippocampal encoding. And novel recombinations of familiar things work too. You don't need to travel internationally to create novelty. A Tuesday morning where you work from a different location, talk to a different colleague, and take a different route home contains more event boundaries than a Tuesday where everything runs on autopilot, even if the component parts are individually familiar.

The goal is not novelty for its own sake. It is event segmentation. Giving the brain enough perceived change that it opens discrete folders rather than filing the week as a single entry.

Sleep is the least optional intervention. Everything else you do to create rich novel encoded experience is contingent on sleep completing the consolidation. Sleeping less than seven hours, consistently, over weeks and months, means a meaningful fraction of what the hippocampus encoded during the day never transfers to long-term storage. The events happened. They were initially tagged. They were simply never saved.

The practical prescription is not sophisticated. Protect slow-wave sleep by going to bed early enough to complete full sleep cycles. The deepest, most consolidation-rich stages of sleep occur predominantly in the earlier cycles of the night. People who sleep six hours are not simply missing one cycle. They are disproportionately cutting the stages where memory transfer is most active.

This is not a wellness recommendation. It is memory infrastructure. The experiences you had today are sitting in temporary storage. Sleep is the process that moves them to permanent storage. Without it, you are living richly and archiving almost nothing.

The third intervention is the hardest. Presence.

The default mode network will wander. That is what it is built to do. The question is whether it is allowed to wander during moments that deserve encoding. The phone during dinner is not a distraction in the colloquial sense. It is a neural gating event. When attention leaves the present and the default mode network takes over, the dopamine signal to the hippocampus quiets. The encoding gate closes. The specific unrepeatable moment happening in front of you stops being tagged for storage. You will not remember the dinner. You were not present for it in the only sense the hippocampus recognizes.

This is not a moral argument about phone use. It is a mechanical one. If the moment matters to you, if you would prefer to have it available later, then the attention required for encoding needs to be present long enough for the hippocampus to do its job. That requires, at minimum, not being somewhere else entirely while it's happening.

Wrapping up

The neuroscience suggests a reframe of how we think about a life well-lived. The conventional measure is duration. How many years. The more accurate measure, for subjective experience, is experiential density. How many distinct, richly encoded memories the brain was actually given material to build.

A long life with few event boundaries, chronic sleep debt, and persistent inattention produces a thin archive. A shorter life with high novelty, good sleep, and deliberate presence produces a dense one. In memory, which is the only place the past exists, the denser archive feels longer, fuller, more inhabited.

Seneca said it was not that life was short but that we wasted it. The hippocampus would say it is not that the years were few but that most of them were filed as one.

The insidious feature of temporal compression is that it is not felt in the present. You do not experience a routine Tuesday as being erased. It feels like a day. It is only in retrospect, when you try to recover it and find almost nothing there, that the loss becomes apparent. By then it is already compounded. The routine that produced one thin year has produced another, and another, until a decade has gone by and the filing cabinet for those ten years holds what childhood held in two. The loss is only visible from a distance, which is precisely when you can no longer do anything about the years that are already gone.

Kierkegaard said life must be lived forward but can only be understood backward. The neuroscience adds a condition he couldn't have known. What you will be able to understand backward is only what you encoded going forward.

The hippocampus is waiting for something new to happen. It will file everything you give it. The question is how much you decide to give it.

TL;DR

Your hippocampus uses novelty as its primary filter for what gets stored as memory. If today looks like yesterday, the brain files them as a single compressed entry. This is why decades vanish. You lived 365 days but your brain archived roughly 40 of them.

Three mechanisms accelerate the erasure. Novelty collapse, where routine periods get compressed because the brain has nothing new to file. Sleep debt, where experiences get encoded during the day but never transfer to long-term storage at night. The default mode network, where attention wanders during moments that deserve encoding, so the hippocampus never tags them in the first place.

The fix follows directly from the mechanism. Novelty creates event boundaries the brain can file separately. Sleep moves them from temporary to permanent storage. Presence keeps the encoding gate open while the moment is happening.

Seneca said it was not that life was short but that we wasted it. He was describing the same thing the fMRI studies are now confirming. The years are there. Most of them just never got filed.