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The heady space between sleep and wakefulness is typically called hypnagogia. As we've written previously, it's a curious, trippy state that has inspired many creative minds, including Beethoven, Edgar Allan Poe and, unsurprisingly, Salvador Dali. But, the mental no man's land actually comes in two varieties: hypnagogia, which transpires before we fall asleep, and hypnopompia, which takes place right as we wake up. Consider hypnapompia the sunrise to hypnagogia's sunset.

The two border-states aren't interchangeable, but experts haven't reached consensus about their precise differences, and often lump them together. "Boundaries between states are always difficult to explore because they're unstable periods, and you need to awake someone fully to study them," said Baland Jalal, a neuroscientist at Cambridge University. Population surveys and research on sleep and neurology, however, have shed some light on what happens when we're drifting off and coming to.  

Hypnopompia and hypnagogia occur at different points in the sleep cycle and appear to facilitate different types of hallucinatory experiences. They've been described as mirror states. But, according to Jalal, the characterization isn't quite right. "Sleep onset [falling asleep] is a slow, gradual phenomenon — it takes 5-30 minutes in the sleep lab. Sleep offset is a more rapid, abrupt phenomenon." Unfortunately, discussions of the two phenomena (in published research and casual online forums) may address them separately or together, or alternate between the two methods without explanation, which complicates the task of teasing them apart. 

Hypnopompic hallucinations are often vivid, immersive experiences and usually a continuation of dream sequences. Hypnagogic hallucinations, on the other hand, tend to be simpler and confined to the visual domain.

Both states are fertile ground for hallucinations. (Note: Technically, true hallucinations must occur during full wakefulness, so hypnagogic and hypnopompic hallucionations are considered "sleep-like experiences." But, for the sake of simplicity, let's just consider them hallucinations.) Often, border-state hallucinations (sometimes called hypnoidal dreams), are associated with narcolepsy, the neurological disorder marked by sudden, irrepressible sleep-attacks, and sleep paralysis, a sleep disorder involving episodes of muscle paralysis and hallucinations. Sleep paralysis is both a hallmark feature of narcolepsy (most sufferers have it) and a stand-alone disorder that affects about 20 percent of the general population. 

In narcolepsy, Jalal explained, hallucinations happen most often during sleep onset (meaning they're 'gogic). For non-narcoleptics, sleep paralysis hallucinations are more likely during hypnopompia (waking up), and often manifest as the sensation of falling or the perception of an external presence in the room, such as the "incubus," the archetypal demon that sits on one's chest during a sleep paralysis attack. Whether during sleep paralysis or not, hypnopompic hallucinations are often vivid, immersive experiences and usually a continuation of dream sequences. Hypnagogic hallucinations, on the other hand, tend to be simpler.

how many people can reliably recall the nature and sleep-cycle timing of their nocturnal hallucinations?

In 1996, UK researchers surveyed 4,972 people aged 15 to 100, who demographically mirrored the general population, about their experiences with hypnagogic and hypnopompic hallucinations. Thirty-seven percent of participants reported frequent hypnagogic hallucinations and 12.5 percent reported frequent hypnapompic hallucinations. Both 'gogic and 'pompic hallucinations were more common for people who also reported insomnia, anxiety and depression. Given the relatively low prevalence of narcolepsy (.04 percent), researchers did not feel that undetected narcolepsy could explain the unexpectedly high frequency of hallucinatory experiences. Still, prevalence estimates can be opaque. It's not clear, for example, if the reported hallucinations occurred during or outside sleep paralysis, or both. Granted, how many people can reliably recall the nature and sleep-cycle timing of their nocturnal hallucinations?  

Researchers also offer explanations of how the two states differ in terms of consciousness. But, Jalal warns, these explanations are hypotheses, not truths to swear by. One hypothesis explains hypnogogia as a period of "rational waking cognition" during which sleepers draw connections between seemingly unrelated experiences and phenomena. This would mean that the hypnagogic mind is capable of logical, self-aware thoughts, such as "I know I'm in a dream." said Jalal. (Some theories of REM sleep say the dreaming mind makes associations the waking mind can't.) 

According to the same framework, self-awareness is absent during hypnopompia, so people cannot recognize their surreal mind-state. Instead, they experience sensations and scenarios from a first-person perspective, as they would in waking life. It's also hypothesized, explained Jalal, that self-aware consciousness progressively fades as we fall asleep, and that intermediary states (i.e. hypna/hypno) have elements of both waking and sleep consciousness. 

As of now, hypnagogia hogs the limelight in discussions of border-state adventures, but people can manipulate both the entrance to sleep and the exit from it. There are two different doors.