Stop us if this sounds familiar: you’re lying in bed almost asleep, your eyes closed and body relaxed, when you realize that for the past few minutes you’ve been watching subtle patterns evolve behind your eyes. You’ve just had a close encounter with hypnagogic visions, a fascinating phenomenon that occurs when you’re on the borderland of sleep. 19th century French psychologist L.F. Alfred Maury coined the word hypnagogia by combining the Greek words hypna (“sleep”) and agogeus (“leader” or “guide”). Hypnagogia is the state that leads us into sleep. Some sleep researchers also employ the word hynopompia for the state that immediately precedes waking, though its accompanying phenomena are essentially identical to those observed in the hypnagogic state.

Hypnagogic visions and associated phenomena are of great interest both to neurologists and dream workers for the window they offer onto the transition between waking and sleeping states of consciousness. The hypnagogic state often generates visual and auditory hallucinations, changes in perception of body size and position, and bizarre ideation similar to the nonsensical ideas you might encounter in dreams. These altered sensory perceptions can be quite vivid, so much so that some dream workers use them to trigger lucid dreams or out-of-body experiences.

While Maury may have first defined hypnagogia as a distinct state, he was definitely not the first to notice it: thinkers as far back as Aristotle have been aware of hypnagogic visions, what he called “the images that present themselves to us in sleep”. Observing and recording one’s hypnagogic visions was part of the occult practices in Europe, including alchemy, and many practitioners of these arts considered hypnagogia a prime source of mystical insights and spiritual exploration. In contrast, some researchers today take the approach that hypnagogia is an epiphenomenon— the brain’s way of clearing its neural circuits of mental static or “clutter”. Even within this framework, learning to become aware of your hypnagogia provides a firsthand view into your brain’s subconscious processes, one that’s much easier to observe than ordinary subconscious dreams.

Hypnagogic visions and other sensations are also useful signposts for aspiring lucid dreamers. Hypnagogia occurs in the state that immediately precedes (or follows) sleep, in which your brain disconnects the sensory input coming from your body and senses. By learning to recognize when you’re experiencing hypnagogia, you will be able to realize that you’ve slipped into the twilight state from which it is easiest to achieve a lucid dream or astral projection experience.

Various sensations can accompany hypnagogia, some of them quite vivid and even scary if you don’t recognize them for what they are. For instance, there’s the phenomenon of sleep paralysis, in which you come fully awake while being unable to move because your body is still basically asleep. However, less alarming hypnagogic phenomena also arise in all five sensory dimensions, any of which you can recognize and even manipulate with practice:

-Visual: hypnagogic visions, either in black and white or color, are the most obvious and widespread aspect of the hypnagogic experience, and can range from simple, shapeless blobs to complicated geometric and even figurative shapes (images of objects, people, landscapes, etc.). These shapes can be still or moving, flat or three dimensional, and may even evolve into short little dream segments called dreamlets. Because human beings are so visually oriented, hypnagogic visions are the easiest of these phenomena to notice and are easily incorporated into a wake-induced lucid dreaming practice.

-Auditory: less commonly, you may seem to hear people talking, music, or simple sounds while you’re lying in a hypnagogic state. As with hypnagogic visions, these auditory hallucinations can range from the simple— buzzing, crashing or ringing— to the complex sounds of speech or even snatches of music or poetry. For some practitioners, inducing auditory hallucinations is an effective route to achieving astral projection: one method is to repeat a familiar word (such as your name) and try to listen for it as if someone else is saying the word.

-Olfactory/gustatory: occasionally, people have experienced smells and tastes as they’re falling into a hypnagogic state. These sensations are much more rare than visual or auditory hallucinations.

-Movement/proprioception: some people, including me, have noticed sensations as though they’re floating or bobbing on water as they’re about to fall asleep. A startling related phenomenon is called the hypnic jerk: a sudden feeling of falling that snaps you awake. This can be accompanied by a dreamlet in which you dream of tripping or falling. Some people also experience alterations in their perception of body size and proportions, which is probably a result of the brain disconnecting sensory information about the position of your limbs as you fall asleep.

Hypnagogia can be useful in training yourself to recognize when you’re falling asleep, but it can also be annoying. For instance, have you ever been trying to fall asleep after a long day of repeating some tedious task, only to find yourself seeing the same motions behind your eyes? If so, it’s a manifestation of what has been dubbed the “Tetris effect”: long-known to restaurant servers and factory workers, this is a kind of hypnagogic vision in which your brain generates a dreamlet of the repetitive task you went through, especially if that task is new to you. Luckily, realizing that you’re going through this Tetris effect can be enough to dispel it: I came back from an almost 6-hour drive recently, and could see the freeway scrolling past me while I tried to fall asleep. Once I became aware of the vision, though, I was able to ignore it, whereupon it quickly faded away.

Though they may sometimes keep us awake, hypnagogic visions are by and large a useful tool for getting to know your own consciousness and what it feels like when you are falling asleep, perhaps to dream!