Sleep researchers refer to that first vague sensation as a “white dream”—and its true nature is a scientific mystery. It’s known that white dreams can occur at any part of the sleep cycle, though they are more likely to occur during non-rapid eye movement, earlier in the night. Sometimes they are explained as a case of simply forgetting what was being dreamed. But some researchers now believe that something much stranger is going on. Rather than reflecting a memory deficit, white dreams might represent a boundary between sleep states, consisting of a basic form of consciousness without detailed sensual content.

If so, for a large part of the night, we really are dreaming of nothing. And probing that fundamental state of being might help us understand the foundations of all other conscious experiences.

The idea that white dreams are due to some kind of lack of memory dates at least to the time of Sigmund Freud, in the late 19th and early 20th centuries. In The Interpretation of Dreams, the father of psychoanalysis argued that dreams express our subconscious desires and anxieties, as the brain’s repressive instincts are relaxed. On awakening, however, this “psychic censorship” could come into full force again by blotting out any fantasies that would be too shocking for the conscious mind to handle. Contentless dreams—now known as white dreams—were the result of this repression, Freud said, but he believed they could be recovered through analysis.

Read: The trippy state between wakefulness and sleep

Freud’s theories of psychic censorship might have fallen out of fashion, but modern neuroscientists have hypothesized that white dreams are rich mental simulations that were indeed simply forgotten, perhaps because the neural activity at night was not sufficient to encode the experience for later recall.

In a 2017 study, Francesca Siclari at Lausanne University Hospital and colleagues at the University of Wisconsin at Madison invited 32 participants to spend a night in the lab while EEG electrodes on the scalp recorded their brain activity as they slept. The team woke up the participants and asked them to record whether or not they had been dreaming in the moments beforehand—and, if so, what they had been dreaming about. When the participants reported white dreams, Siclari and her colleagues found that the front and center of the brain—normally implicated in memory encoding—lacked the characteristic high-frequency activity that was found with remembered dreams.

The brain, in other words, didn’t appear to be running the machinery to create memories in the first place. So why would humans evolve to have these vivid nighttime experiences if so many of them are forgotten? “Maybe forgetting is a natural part of the function of dreaming,” says Tore Nielsen at the University of Montreal, who wasn’t involved in the study. It’s possible that dreaming might play some important role—such as processing the day’s emotions—but the contents are then forgotten to avoid clogging up our memories with fictitious events.