Sleep Hallucinations: Why Do They Happen?

Have you ever experienced a dream so vivid, it might as well have been real? Do you recall a time where you’ve seen spiders crawling up your walls, a monster at the end of your bed, or even heard faint singing when dozing off? If you said yes to either of these questions, you’re not alone. These are parasomnia hallucinations— hallucinations that occur as a result of abnormal activity of the nervous system.

There are two main types of these hallucinations: hypnagogic and hypnopompic. What’s the difference between the two? Hypnagogic hallucinations occur between the transition from wakefulness to sleep, while hypnopompic hallucinations occur from sleep to wakefulness. These hallucinations are more common than you think, it was found in a recent study that 37% of a sample of 4972 people reported experiencing hypnagogic hallucinations and 12.5% reported hypnopompic hallucinations.You may remember being in a hypnagogic state when you doze off on the couch and the sound of the TV slowly turns into a distorted jumble of words and sounds, or when you listen to a song so much that you may catch it in your dreams. Sometimes, if you have repeatedly performed an action like playing a video game, this may make a surprise appearance in your hypnagogic hallucinations; this is known as the “Tetris effect,” where one repetitive act is manifested in parasomnias or dreams.

Occurring in bursts between five to ten minutes, hypnagogic states are the most common form of sleep hallucination. Hynagogia occurs as a natural part of sleep for some individuals with heightened brain activity, especially those frequently in high-stress environments, those who experience repetitive situations, or even after viewing very stimulating media. Though hypnagogic states are not entirely understood, it is widely believed that the transition between types of brain waves from wakefulness to sleep are the culprit. When falling asleep, the transition begins with alpha waves, which are released during times of relaxed consciousness, to theta waves, which occur during early sleep, to delta waves, which are characteristic of deep sleep. This transition normally occurs in the process of falling asleep, however, when hypnagogic states occur, the distribution of these brain waves is uneven, and the brain begins to “turn off” specific areas of the brain faster than others. This leads to certain regions of the brain such as the occipital lobe or the cerebellum to stay “on,” forming visual hallucinations and certain falling sensations. 

Hypnagogia’s ugly sibling, hypnopompic states, are often described as much more unsettling, scary and vivid, and can be difficult to distinguish from reality. Hypnopompia is the transitory period between sleep to wakefulness, and is far less common than hypnagogia. Unlike the faint and often abstract images that individuals experience in hypnagogia, those who experience hypnopompic states report solid and seemingly tangible figures that they are able to touch, smell, or speak to.

Henry Fuseli’s “The Nightmare” is believed to be one of the earliest 

depictions of a hypnopompic state.

Unlike the natural and lulling quality of hypnagogic hallucinations, hypnopompic states are considered more like little brain “glitches” that occur as a result of abnormal electrical pulses in non-REM sleep stages, or deep sleep, where dreaming does not occur. Why is this important? In REM sleep, the brain experiences intense electrical activity from the brain stem, which leads to dreams, however, once your REM cycle ends, you enter NREM (non-REM sleep) which is characterized by virtually no electrical activity in the brain stem; this is where you brain is “quietest” and is also known as deep sleep. Since there are no dreams occurring normally in NREM, it is often the most common stage to wake up in, however, in some circumstances, the brain stem sends out slight pulses of electrical activity during NREM, making two things happen A) dream synthesis mechanisms are activated (just as in REM) and B) your brain sends signals that something is abnormal and makes you begin the process of waking up. All of this combined and you begin the process of waking up, but your dream mechanism is still activated, making you hallucinate in a groggy, in-between mode. Not only this, but a link was recently found between these states of hypnopompic and deficits in serotonin and dopamine, giving many of these hallucinations an unpleasant and negative theme.

Stages of Sleep, the Blue is non-REM sleep

All in all, hallucinations such as these can be, well, unpleasant. So what can you do to reduce hypnopompic and hypnagogic hallucinations? Scientists believe that reducing intense stimuli a few hours before bed can be effective in reducing the frequency of such states, which means put down your phone, stop watching horror movies, and start meditating a little bit before bed. If these hallucinations persist, it could be a sign of more serious disorders, so talk to a sleep specialist if need be.