Dom Cobb, a professional thief capable of entering people's dreams to steal information, wakes up with a start. He knows he's only been asleep for a few minutes, but within his dream, as vivid as life itself, an entire week has passed. With this premise, the film Inception (2010) won four Academy Awards and made the perception of time one of the great reflections of mass culture.

However, Christopher Nolan's fiction didn't invent any philosophical debate; in reality, it represented (and exaggerated) a common human experience. Like Dom Cobb, we've all woken up some morning with the feeling of having spent several days traveling, making decisions, and experiencing the narrative twists and turns of our minds. Or, conversely, sometimes a single blink can fill an entire night. Ultimately, what's fascinating is realizing that something doesn't quite fit between the clock on the nightstand and the "lived" experience. What happens to time when we dream? Does it speed up? Does it compress? Or is it we who, upon waking, retrospectively reconstruct a duration that never truly existed?

In a study from the 1960s, people asleep in REM sleep (the phase in which dreams are most vivid) were exposed to timed stimuli. When they awoke, the participants were able to distinguish quite accurately between brief intervals, such as 30 seconds or three minutes. This result suggested that, at least under certain conditions, the subjective time of sleep can correspond well with physical time. A few years later, researcher Stephen LaBerge started from the premise that, during REM sleep, the body is practically paralyzed, but the eyes continue to move. Taking advantage of this, LaBerge trained a sequence of eye movements to "communicate" from within the dream. While asleep and aware that he was dreaming, he would point with his eyes, mentally count to ten, point again, and repeat the process. The polysomnographic recording showed that those ten seconds of dreaming lasted practically the same as ten seconds of waking life. At least in this case, the internal clock seemed to function the same. However, other studies, and especially many dreamers, recount stories of several days condensed into a single moment of sleep, something that seems to contradict previous results.

This is where the hypothesis that time is not measured, but constructed, comes into play. From a neurocognitive perspective, Jason W. Brown's microgenetic model proposes that all conscious experience—and sleep is no exception—emerges as a process in which multiple mental states are activated simultaneously. In the specific case of dreams, these activations are superimposed layers of emotional, sensory, and narrative fragments without a common temporal axis. It is precisely in this intertwining that the sensation of "duration" arises. In other words, when the brain reactivates disconnected memories, the sleeping mind generates an impression of continuity that depends solely on the internal coherence of the experience itself; that is, it does not depend on the minutes or seconds that have passed. This mechanism also appears in waking life. For example, five minutes in a boring meeting can seem like an eternity; but an afternoon spent in a state of flow on a creative task can evaporate without us noticing. Therefore, the perception of time in dreams would be an extreme case of a general human capacity, in which psychological time is not a fixed magnitude.

Currently, some analyses of large dream databases show that multisensory richness and emotional intensity can increase the coherence of perceived time within the dream. The more senses and emotions that come into play, the more "lasting" the episode seems, even if physiologically it was brief. Furthermore, studies on attention and lucid dreaming indicate that consciousness and intention can modulate how time feels while we dream, suggesting that the experience of time is relatively malleable. For all these reasons, talking about "how long" a dream lasts is almost misleading, because what we remember as duration is, in reality, the trace of an intensely condensed cognitive process.

The philosopher Jennifer Windt posits that dreams are immersive mental simulations, situated halfway between perception and imagination. They are experienced as real, but are not anchored to the temporal mechanisms of wakefulness, hence their elasticity. Thus, dream time is convincing while it occurs, but eludes any attempt at measurement once awake.

Ultimately, Nolan's reflection on time in dreams reveals that, despite the clues provided by science and the humanities over the last few centuries, despite all the clocks and calendars in the world, we still don't fully understand the nature of time perception. However, the limited evidence available to date suggests that dream time is not a flawed version of waking time, but rather a distinct mode of consciousness.

The original article was published in Spanish at Ethic.es