From recalling important deadlines to remembering meaningful conversations, memory shapes nearly every aspect of daily life. It allows people to learn from the past, function in the present, and prepare for the future. Despite how reliable memory may seem, it is more fragile than most realize. Even small changes in physical or mental well-being can influence how information is stored and later received. While it is a natural cognitive process to forget, several factors can accelerate the rate at which it occurs. Many people are familiar with medical conditions that are associated with memory loss, like dementia and Alzheimer's disease. However, memory can also be impaired by less commonly discussed causes, including psychological trauma and sleep deprivation.
Most people have trouble recalling memories during the first few years of their lives due to a phenomenon known as childhood amnesia. Memory formation relies on the hippocampus, a structure located in the brain's temporal lobe that is connected to episodic, or personal, memories [1,2]. However, the hippocampus doesn’t fully develop until at least the age of seven, meaning memories formed before then can be difficult to recall later in life [1].
As people grow older, the memories that form and remain can be categorized into three main categories: explicit memories, implicit memories, and short-term working memory. Explicit memories concern general information or daily events, and mainly involve the hippocampus. On the other hand, implicit memories, concerning motor memories, rely on the cerebellum and the basal ganglia. Lastly, short term memories are created in the prefrontal cortex [2]. In all of these parts of the brain, specific groups of neurons create connections between each other called synapses. These circuits of connections can be made stronger or weaker depending on when and how many times they’ve been activated in the past. Memories are formed based on reactivation of certain combinations of neurons. Additionally, in adults, new neurons can be created through a process called neurogenesis, which can improve the formation of new memories. Frequent activation of certain neurons can strengthen the bond between them, solidifying those memories, but less consistent activation weakens the connections, which can lead to memory loss [2].
Memory loss can be grouped into passive and active forgetting. Active forgetting is a mechanism that intentionally weakens memories. It can happen due to new information that is prioritized over old memories , acceleration of memory decay, voluntary suppression of unpleasant memories, or the recalling of one part of a memory to suppress another. A new concept, intrinsic forgetting, removes old or "irrelevant" memories by creating specific forgetting cells to actively degrade memory to increase brain efficiency and adapt to new information. On the other hand, passive forgetting occurs due to more “natural” causes, such as a loss in context cues, confusion with similar memories, or natural decay over time [3]. The cognitive process of the brain usually peaks during one’s 20s. Afterwards, it is normal for one’s processing speed to slow down with age and for the amount of content in the working memory to diminish [4].
Sleep is an active process that stabilizes memories, so consistently falling short on sleep can lead to memories not being stored in the brain effectively. Sleep supports memory consolidation by allowing the hippocampus to “replay” new experiences and transfer them to the cortex, where long term storage occurs. Without sleep, this process breaks down, and new information is unable to be stabilised [5]. Additionally, different stages of sleep are important for active communication to occur between different brain regions. This process can be disrupted when there is a lack of sleep, which means the new information learned never moves into stable long term storage, making the brain more prone to forgetting what it learns. [6] Further, when the brain is deprived of sleep, the orexin system — which regulates wakefulness — becomes overstimulated, interfering with the production of new neurons in the hippocampus. Since neurogenesis is vital for forming new memories, the inhibition of doing so weakens the brain's capacity to learn and store information. It was found that sleep deprivation leads to increased expression of certain chemicals and processes like orexin-A and PLCβ1 and ERK1/2 phosphorylation. This alters signalling pathways in the brain, ultimately creating a hostile environment for new neuron growth [7].
Trauma also has a huge impact on memory loss. Physical traumas, like strokes or concussions, can impact the function of memories, especially if they affect brain regions crucial for memory processing, like the hippocampus. In the US, it is estimated that around 3.8 million concussions occur each year in the US. [8]. Memory loss is a common concussion symptom, usually lasting a few weeks proceeding the concussion. However, if the memory loss lasts longer, it becomes a sign of post-concussion syndrome (PCS). While it is not known what causes PCS, some factors that put people at risk for PCS include age, anxiety, and prior headaches or brain injury [9]. These traumatic brain injuries can result in short-term memory decline, challenges in remembering tasks, or memory loss based on the severance of the injury incident [10].
Apart from physical traumas, emotional or psychological traumas can also affect one’s memory. Dissociative amnesia is a defense mechanism when one’s brain purposely blocks out information, creating “gaps” in their memory due to distressing or traumatic experiences. In most cases, the memories still exist but are inaccessible. There are two types of amnesia: Retrograde, which affects finding old memories, and anterograde, which blocks out the formation or storage of new memories [11]. Post-traumatic stress disorder (PTSD) is a mental health condition that is caused from traumatic events, like a war. The symptoms—varying from intrusive memories, avoidance, depressive thoughts, and an increase in emotional and physical reactions— can appear within a few months after the event, or sometimes don’t occur until years later. Intrusive memories can appear in forms of flashbacks, dreams, and more, bringing back the traumatic memories from the event [12].
Some drivers behind memory loss are instead closely associated with increasing age, such as dementia. Classified as a disease that affects memory, thinking, and the ability to perform daily activities, dementia can be caused by a multitude of factors that destroy nerve cells and damage the brain. For example, smoking, depression, and having high blood pressure can all damage nerve cells, and lead to deterioration in cognitive function [13]. Dementia can be caused by many diseases, the most common being Alzheimer’s disease. Alzheimer’s occurs when there is a build up of amyloid and tau, which are substances in the brain that can cause misfolds in tissues around it. When there is a build up of these substances, they clump up and form structures called plaques and tangles, which makes it harder for the brain to work properly [14].
Another form of dementia is Lewy body dementia (LBD). LBD is a progressive disease, meaning symptoms start slowly and get worse over time. LBD is caused when alpha synuclein, a protein that plays an important role at the synapses, forms into clumps in the neurons. This causes neurons to work less effectively and eventually die. LBD affects many of the brain regions, including the cerebral cortex, limbic cortex, hippocampus, and brainstem. By affecting these areas, this also affects the functions of these parts of the brain. This includes information processing, perception, emotions, forming memories, and regulating sleep [15].
Currently, researchers, like the team from Case Western Reserve University, University Hospitals (UH) and the Louis Stokes Cleveland VA Medical Center, are working to find ways to revert the supposedly irreversible damages to memory loss caused by diseases. This specific team has recently discovered a possible way to reach a full neurological recovery from Alzheimer’s disease by maintaining normal levels of a central cellular energy molecule, NAD+ [17]. Studies similar to theirs are being conducted all over the world, working to one day create a world where everyone can live their entire lives with an abundance of memories to look back to.