How Memories Are Made and Why They Fade

There are many normal, non-scary reasons why we forget things—whether it’s the name of that dog-walking neighbour, a friend’s birthday or where we parked the car. Sometimes we’re moving too fast, multitasking along the way. Other times, our lives are filled with too much stress and not enough sleep.

Of course, there are also more serious, and understandably concerning, reasons behind forgetfulness—ones that often spark worry. With constant discussions around dementia and personal experiences with loved ones, it’s natural to be concerned.

To better understand memory, occasional lapses, and cognitive function, here’s a look at how memories form in the brain and what happens when recall starts to slip.

How a memory is formed

Let’s say you walk in the door and put your keys down next to the kettle. “When you do that—when you have any experience—a specific pattern of activity happens in neurons (nerve cells) and particular neurons are activated,” says neuroscientist Tara Tracy.

“The strength of that neuronal activity will determine how well you remember it later. To form a memory, neurons need to be strongly activated, followed by a plasticity effect—a small change in the brain.” Plasticity refers to the brain’s ability to create new neural pathways and modify existing ones in response to behaviour and environment.

Now, imagine that while placing your keys down, your mind is elsewhere: you’re thinking about stopping at the petrol station, realising the floor needs cleaning, wondering if someone picked up the dry cleaning—all while putting away groceries and checking a text.

In this scenario, Tracy explains, the neurons involved in forming that memory are only weakly activated. As a result, you’re far more likely to forget where you placed your keys. “A weakly activated neuron won’t encode the information as effectively. Without strong activation, the plasticity effect won’t occur, making recall more difficult.”

Tracy’s research focuses on this “small change in the brain” that allows memories to form. “We believe this happens at the synapses, the connections between neurons,” she says. “When neurons activate during an experience, synapses within a certain brain region strengthen. That strengthening is what encodes the memory.”

Lifestyle and memory

The way you go about your daily routine has a direct impact on memory. Factors like sleep, exercise, and stress all play a role in how well your brain stores and recalls information.

Sleep is critical for memory storage, though researchers are still working to understand exactly why. Those quiet hours give the brain time to consolidate memories and clear toxins and waste products that build up throughout the day.

Exercise—particularly moderate to vigorous activity—has been shown in a large, long-term study to improve memory and overall cognition. It helps the brain form stronger connections between neurons, supporting cognitive function. Guidelines recommend at least 2.5 hours of moderate-intensity physical activity or 1.25 hours of vigorous activity per week, along with muscle-strengthening exercises. Even short bursts of movement can add up.

Stress has been linked to memory storage and retrieval issues in multiple studies. When stress levels rise, the body releases cortisol, a hormone that affects the brain regions responsible for processing and recalling memories. High cortisol levels over time can make it harder to concentrate and retain information.

When cognition declines

Some level of age-related cognitive decline is a normal part of getting older, says neuroscientist Tara Tracy. Everyone experiences it to some degree, though the extent varies from person to person.

According to the Mayo Clinic, another stage is mild cognitive impairment (MCI), where people may struggle with memory, finding the right words or making sound judgments, but can still manage daily life. They, or their loved ones, might notice difficulty following conversations, tracking a film’s plot or remembering familiar places. Unlike dementia, MCI doesn’t have a single cause and may improve or worsen over time.

Dementia, however, is a different story. Alzheimer’s disease is the most common form, accounting for 60–80% of cases, according to the Alzheimer’s Association. Scientists worldwide continue working on ways to prevent, treat and better understand this condition.

Tracy and her team study memory loss in Alzheimer’s disease and frontotemporal dementia, aiming to understand the differences between normal cognitive ageing and neurodegenerative diseases. “Everyone experiences normal age-related cognitive decline, not just those with a disease,” she explains. “The question is, what are the underlying mechanisms of cognitive ageing versus Alzheimer’s disease? Are they the same or different?”

Research has shown that Alzheimer’s disease is largely driven by toxicity in the brain, caused by a build-up of two proteins: beta-amyloid and tau. “This build-up doesn’t necessarily happen in people who only experience age-related cognitive decline,” Tracy says, suggesting the mechanisms behind the two conditions are distinct.

Still, there are similarities. Tracy’s research indicates that in both normal cognitive ageing and Alzheimer’s, neurons can still be activated in the early stages of memory loss, but synapses struggle to encode new information. “There’s still neural activity, but the synapses can’t encode it. That’s a separate mechanism,” she explains.

Since Alzheimer’s is a progressive disease, people can live with it for five to ten years, sometimes longer. “In the early stages, people’s lifestyles are affected, but they still recognise family members and manage certain tasks. It’s not yet severe dementia,” Tracy says. In later stages, when severe dementia sets in and people no longer recognise loved ones, neuron loss becomes significant. “At that point, the neurons are dying and there’s little or no activity left to even start encoding memory,” she adds.

This highlights why researchers are focusing on early-stage interventions. “The early stages of the disease are where we think it’s the most treatable,” Tracy says. There’s optimism in the scientific community that advancements will bring greater understanding, leading to better treatments—and, perhaps one day, prevention.