A recent proof-of-principle study, led by Noreika, intensively studied a single individual as he repeatedly transitioned into sleep while the brain’s electrical activity was recorded using EEG scalp electrodes. The would-be-sleeper was asked to press a button when he experienced an intrusive thought or image, and to verbally report it to the sleep researchers. The descriptions were pleasantly bizarre: “putting a horse into a sort of violin case and zipping it up,” “the phrase learning to consume consciously from a master,” “visual image of a curled up music manuscript.”

The electrical activity of the brain became steadily more predictable the longer the person lay still—something that’s entirely normal for sleep entry. Unexpectedly though, the hypnagogic intrusions were preceded not by sudden bursts of complex brain activity, like sparks in a fading candle, but by sudden changes to a more orderly brain state. Noreika is working on the hypothesis that when we enter sleep, the brain steadily dismantles the models and concepts we use to interpret the world, leading to moments of experience unconstrained by our usual mental filters.

This is intuitively appealing and would fit with one of the most curious aspects of the hypnagogic experience: Our thoughts can stray towards tumbling horses, zips and violins but they also can seem completely unremarkable, and indeed, entirely reasonable, until we are jolted from our reverie. Only at this point do they seem odd or out-of-context.

The difficulty that people have detecting the strangeness of these experiences prompted psychologists Clemens and Jana Speth, both at the University of Dundee, to examine reports of hypnagogic intrusions. As Clemens says, the two were hoping “to develop a timeline that shows what elements of consciousness decline or emerge as people drift into sleep.” Applying linguistic analysis to data from a sleep lab, they found evidence that reflective thought—the ability to evaluate ongoing experience—declined quickly during the hypnagogic state while thoughts about physical interaction with the imaginary world increased, indicating a change in the structure and not just the content of conscious thought.

Similarly, by comparing the hypnagogic state to REM dreaming, a 2013 study by the same researchers confirmed the long-noted observation that while dreams often feel fully immersive, hypnagogia tends to be experienced as if we were passive observers—with the hallucinatory thoughts and images occurring as a projection on our existing sense of reality. (In a famous passage on hypnagogia in Oliver Twist, Dickens wrote of “the visionary scenes that pass before us.”) The fact that our sense of immersion and reflective distance from our own experience do not always co-occur during sleep may suggest they also have different roles in waking consciousness.