Mark Makela / Reuters

Where do our minds go at night? For more than a century, discussions of dreams have tended to revolve around the interpretation of our dreams’ contents. Do they reflect our unconscious anxieties? Are they an attempt to simulate threats, training us to cope with future challenges? Or are they simply the result of our mental housekeeping, as the sleeping brain reactivates our memories and processes them for long-term storage? In each case, the focus has been on the more immersive, surreal flights of fancy that occupy the sleeping brain.

Yet our most puzzling dreams may not have contents at all. Have you ever woken up with the certainty that you had just been dreaming, yet you were unable to recall even a single detail of the scene your mind was playing out? Various sleep studies have found that approximately 30 percent of the time, participants wake up with the sensation that they have been dreaming about something, yet when they are asked to describe the experience, they draw a complete blank. This is a distinct experience from waking up and having no sense of having been dreaming at all, which occurs about 20 percent of the time, or the rich narratives found in the other 50 percent.

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.

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.

Memory problems alone, however, do not appear to be the whole story. In a new paper for Sleep Medicine Reviews, Peter Fazekas of the University of Antwerp and colleagues instead suggest that white dreams are better understood as a diminished form of consciousness. According to this hypothesis, white dreaming is a bit like watching a badly tuned TV, with the volume muted: The sleeper really is dreaming, but the signal is too weak to establish any definite details beyond the vaguest impressions.

Fazekas’s previous research focused on the variations in waking consciousness, such as the vividness of a sensory experience. In so-called masking experiments, for example, researchers quickly flash one image, “the target,” before the participants’ eyes, followed by another picture, “the mask.” Sometimes the participants have a clear impression of the target—a cat, say—while at other times its presentation is too quick for conscious perception; they only see the mask.

Between those extremes, however, many participants report a vague sense of having seen something, without being able to give the details of what it is. This sense of vividness—or lack of it—usually correlates with activity in the posterior regions at the back of the brain. The greater the high-frequency activity in this area, the richer and more detailed the experience, while muted activity corresponds to the weaker impressions.

Perhaps, Fazekas hypothesized, white dreams are similar to those minimal forms of conscious awareness. Working with Georgina Nemeth at Eotvos Lorand University in Hungary and Morten Overgaard at Aarhus University, he took another look at Siclari’s data to see whether this was true. They noticed that Siclari’s statistical analysis had unintentionally obscured some potentially important differences in the posterior brain activity between white dreams, remembered dreams, and the sensation of having not dreamed at all. Sure enough, a reanalysis of the raw data suggested that white dreams do indeed reflect a striking reduction in that posterior brain activity, compared with remembered dreams, but still greater activity than when participants report no dreaming experience at all.

The reduced frontal and central activity that Siclari observed would naturally follow from this, Fazekas believes, since those regions would have little information to encode into a memory. “For those areas to turn on, so to speak, you need an intense experience, which you don’t have in the white dream,” Fazekas says.

Siclari agrees that Fazekas offers a feasible interpretation of her data, though she believes that the reduced recall is still the primary distinguishing feature of white dreams. When prompted to dig deeply into their memories, she says, some participants were later able to draw details from those apparently content-less experiences, which suggests that in at least some cases, it is purely a failure of recall.

Other researchers have responded warmly to Fazekas’s new paper. “I was thrilled to see white dreams, which are an often-neglected topic, get so much attention,” says Jennifer Windt at Monash University in Melbourne, Australia.

One very real possibility—supported by both Siclari and Fazekas—is that white dreams may in fact comprise a whole spectrum of experiences, the nuances of which may have been lost as the scientists averaged the data across participants. Some white dreams may be vivid, cinematic visions that are simply forgotten, as Siclari suggests, while others may be the kind of vague, gist-like experiences proposed by Fazekas. At the extreme, some white dreams might be completely contentless, containing only “the experience of the passage of time, of an indeterminate duration,” according to Windt.

Studying those particular cases could give us a view of the “simplest forms of subjective experience that exist,” Windt says—something that is “perched on the border between unconscious sleep and more complex and dreamful experiences.” She points out that experienced meditators regularly report a “‘pure,’ nonconceptual awareness” in sleep in which they are conscious of being asleep, but lack any specific thoughts or images. Further research, she hopes, might help verify those descriptions and compare the neural activity with other participants’ white dreams to see whether there is any overlap with this mysterious state.

“Pure consciousness” can sound like a New Age buzzword, but philosophers and neuroscientists are coming to view it as an important concept. You could think of consciousness as a Fabergé egg: Once you peel away the outer layers, you are left with the most fundamental state of conscious existence—the core of our mental world. Finding ways to strip down our mental activity to this has been incredibly difficult, but this latest research suggests that white dreams could offer one important entry point to explore that state and to understand the starting point of all thought and feeling.

White dreams might appear meaningless, but for scientists probing the mysteries of sleep and consciousness, they are rich with possibility.

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