Nick Pelios
Freediver, Creator
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Nick Pelios
Freediver, Creator
Every freediver knows the sensation of time stretching as they descend. Certain details become hyper-vivid: the shimmer of light on the thermocline, the texture of your wetsuit against your skin. Yet, when the dive is over, entire stretches of time can seem to disappear. You might remember a sound, a flash of movement, or a feeling of peace, but not the full sequence of events. This uneven pattern of memory is not a quirk of personality. It is a direct result of how the brain encodes information when oxygen is limited.
During a freedive, oxygen saturation can fall to levels that would trigger medical alarms on land. Peripheral tissues enter a state of controlled hypoxia, and the brain must decide what to preserve, what to suppress, and what to let go. The hippocampus, amygdala, and prefrontal cortex are all involved in this triage. Rather than storing information evenly, the hypoxic brain prioritizes emotionally charged, survival-relevant, and sensory-rich data while downregulating background processing. This adaptive response, shaped by evolution and refined through training, can explain why freedivers often report intense, almost spiritual recollections of certain moments underwater, while struggling to recall the rest.
Under normal oxygen conditions, the hippocampus functions like a high-speed relay station, integrating information from sensory cortices and binding it into coherent episodes. It depends heavily on aerobic metabolism to maintain synaptic plasticity and support long-term potentiation, the process by which neural connections strengthen to encode memories. When oxygen availability drops, the hippocampus is one of the first brain regions to feel the impact. Its neurons are metabolically expensive, and their activity is quickly curtailed to conserve resources for essential functions.
This reduction in hippocampal activity leads to selective encoding. Instead of creating a continuous, detailed record of events, the brain switches to a sparser mode, storing only fragments deemed relevant or salient. Laboratory studies have shown that mild hypoxia can impair declarative memory formation while leaving emotional and procedural memory relatively intact. This aligns with what many freedivers experience: a vivid memory of the emotional tone of a dive, or of specific sensations, but gaps in the factual timeline. It is not that the diver was inattentive; it is that the hippocampus was working with limited energy, encoding in shorthand rather than full paragraphs.
Interestingly, repeated exposure to hypoxia through training can modify hippocampal resilience. Some evidence suggests that intermittent hypoxia can stimulate angiogenesis and mitochondrial efficiency, potentially improving the hippocampus’s ability to function under low oxygen. While research on freedivers specifically is still emerging, animal models and studies of high-altitude populations hint that the brain can adapt to these conditions over time, preserving more cognitive function during hypoxic episodes.
If the hippocampus is the archivist, the amygdala is the gatekeeper. It flags experiences with emotional significance and tells the rest of the brain to store them more robustly. Under hypoxia, the amygdala becomes even more influential. While hippocampal activity declines, the amygdala remains relatively active and can even become hypersensitive. This means that emotionally intense or novel stimuli are more likely to be encoded than neutral ones.
Freedivers often describe moments of profound connection or awe underwater, sometimes even during routine dives. These emotional peaks are not accidental. They are the result of the amygdala tagging certain experiences as significant in a state where the brain has limited resources. The shimmering movement of a school of fish, the sudden change in water temperature, or the feel of a pressure shift at depth can all trigger a cascade of emotional and physiological responses that the amygdala flags as important. Later, these become the core memories of the dive, replayed with striking clarity.
From an evolutionary perspective, this makes sense. Our ancestors relied on detecting and remembering emotionally charged events, predators, critical environmental cues, moments of intense beauty or danger. Hypoxia may amplify this selective process. By prioritizing emotional salience, the brain ensures that crucial experiences are stored even when energy is scarce. For freedivers, this means that the ocean’s emotional tapestry is often remembered more vividly than the objective sequence of actions.
Another key player in memory encoding under hypoxia is the prefrontal cortex, responsible for executive functions, planning, and sustained attention. It is one of the brain’s most oxygen-hungry regions. As oxygen levels drop, its activity diminishes, leading to a narrowing of attentional bandwidth. Freedivers often describe this as a sense of tunnel focus or flow, where peripheral thoughts fade and awareness locks onto the present moment.
This shift has both benefits and drawbacks. On the positive side, reduced prefrontal interference can enhance sensory immersion and facilitate intuitive motor control. Many athletes, not just freedivers, seek this state because it allows them to act fluidly without overthinking. On the negative side, the decline in prefrontal monitoring can reduce working memory capacity and impair the brain’s ability to encode contextual details. In practical terms, this is why a freediver may remember the feeling of gliding through a thermocline but forget the exact depth or the order in which events unfolded.
There is also evidence that hypoxia alters neurotransmitter dynamics in the prefrontal cortex. Dopamine levels can shift, contributing to the altered sense of time that many freedivers report. Minutes may feel like seconds, and a single image or sound can expand in perceived duration. This temporal distortion is intertwined with memory encoding, as the brain compresses or stretches experiences depending on salience and available cognitive resources.
Training can partially mitigate these effects. Freedivers who engage in regular breath-hold practice may develop better tolerance for prefrontal deactivation, maintaining functional attention for longer under hypoxia. Techniques such as mental rehearsal, task simplification, and consistent dive routines can reduce the cognitive load on the prefrontal cortex, allowing scarce oxygen to support essential processes more efficiently.
The way freedivers remember their dives is not random. It is the result of a sophisticated, adaptive system shaped by both biology and practice. Under hypoxia, the hippocampus encodes selectively, the amygdala amplifies emotional salience, and the prefrontal cortex narrows focus. Together, these changes produce memories that are emotionally intense, sensory-rich, and fragmented. With training, the brain can refine its response, improving resilience and perhaps expanding the amount of information it can encode under oxygen stress.
These insights have practical implications. Freedivers who understand how hypoxia shapes memory can use this knowledge to refine their mental preparation and debriefing. For example, immediately reviewing a dive while still at the surface can help transfer fragile hippocampal traces into more stable long-term storage before they fade. Visualization and journaling after sessions can also consolidate memories, integrating emotional impressions with factual details. Instructors might consider incorporating memory training techniques into their programs, helping divers become more aware of what their brains are likely to retain or discard.
Beyond performance, there is a deeper layer. The selective nature of memory under hypoxia may contribute to the unique way freedivers relate to the ocean. The moments that are encoded are often those that evoke wonder, connection, or alertness. These become the stories divers tell, the sensations they chase, the internal compass that draws them back to the water. Far from being a flaw, this selective encoding is part of the freediving experience itself. It shapes how individuals build their personal mythology of the sea, blending fragments of sensory memory into a narrative that feels timeless.
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