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Have you ever found yourself seeing images right before falling asleep? You might be experiencing hypnagogia.

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Sources:

https://www.theatlantic.com/science/archive/2016/04/deciphering-hypnagogia/478941/
https://www.scientificamerican.com/article/tetris-dreams/
https://www.sleepassociation.org/patients-general-public/hallucinations-during-sleep/
http://www.merckmanuals.com/professional/neurologic-disorders/sleep-and-wakefulness-disorders/narcolepsy
http://www.sciencedirect.com/topics/neuroscience/hypnagogia
https://www.ncbi.nlm.nih.gov/pubmed/8894197
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4173919/
https://www.britannica.com/topic/dream-sleep-experience#ref423110
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https://books.google.com/books?id=q_vpBAAAQBAJ&pg=PA117
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https://pdfs.semanticscholar.org/09d8/95b85d772fb505144969310255c0cbdc74a7.pdf

http://www.academia.edu/33150155/Replaying_the_Game_Hypnagogic_Images_in_Normals_and_Amnesics

http://www.businessinsider.com/what-is-a-hypnic-jerk-2014-5
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https://www.sleepassociation.org/patients-general-public/sleep-myoclonus/
http://www.bbc.com/future/story/20120522-suffer-from-sleep-shudders
(Intro)

Have you ever played a video game for so long that when you finally put down the controller and are maybe falling asleep, you see images of the game flash before your eyes?  This has happened to me a bunch of times personally.  One time, I had played so much Tetris and I was writing a paper in college that I got surprised that the words on my screen weren't falling down.  I was like, why are they all just staying there?  I was expecting them to like, start--I was very tired.  

These hallucinations can be other things, too, like jigsaw puzzles or apple picking, and they happen in the weird limbo between wakefulness and sleep called hypnagogia.  In fact, lots of strange things can happen in this state, like twitching or something called exploding head syndrome.  Don't worry, no brain bits are involved in that one, but it can still get pretty trippy.  

Scientists don't know a ton about hypnagogia, but they are learning.  They consider it to be an altered state of consciousness, not unlike an LSD trip.  The hypnagogic state only lasts about 10 minutes as you transition from being awake to being asleep, although sometimes it's called the first stage of sleep, and during this period of drowsy pre-slumber, you can hallucinate.  

You might feel like you're falling, flying, or floating.  You might hear short phrases or sounds or see anything from amorphous shapes and colors to clear images of people.  Now, this might seem like dreaming but it's a little different.  Most dreams happen during Rapid Eye Movement, or REM sleep, where your brain is much more active than during the rest of your shut-eye, and dreams are usually pretty vivid.  You're part of some kind of action or story like going to outer space in a hot air balloon to meet a clown that sounds like Sir Patrick Stewart.  

But hypnagogic states are more like watching snippets of films in an experimental art gallery.  Salvador Dali even famously used hypnagogic imagery to inform his art, disrupting his naps to get inspiration.  It's not always the case, but the imagery can come from something mundane or repetitive you were doing while you were awake, like playing Tetris.  That's why this hallucination phenomenon is sometimes called the Tetris Effect.

Scientists don't know why these hallucinations happen because not everyone gets them, and they can be a symptom of disorders like narcolepsy, which is basically extreme sleepiness, but one idea is that some parts of your brain take longer than others to fall into sleep mode.  As you're drifting off, your brain's electrical pulses or brainwaves start to slow down.  They shift from moderately fast alpha waves, which are typical of being relaxed but awake to slower theta waves of early sleep to even slower delta waves of deep sleep, but the slowing down isn't even.  EEG measurements of brainwaves suggest that your brain puts on the brakes faster in the front than in the back, and that means that the parts of the brain in charge of vision or balance could be active for a bit longer, which might explain why so many hypnagogic hallucinations are visual or include weird sensations like falling, and because your frontal cortex is snoozing during this in-between state, it can't help you sort out what's going on like it normally does.

One of the most frightening things that can happen during this drowsy lull is hearing a loud exploding sound like a cymbal crash or gun shot.  Sometimes there's also a flash of light.  This is called exploding head syndrome and while it doesn't hurt, it can be so scary that it wakes people up from their half asleep state.  It's not clear whether this is just a specific type of hypnagogic  hallucination or whether it's something else.  Some neuroscientists consider it a sleep disorder and speculate it might be from tiny seizures or sudden bursts of electrical activity in the brain.  Fortunately, most people who have this problem aren't as bothered by it once they find out that it's not a sign of something more serious like brain cancer.

Another thing that can happen during hypnagogia is the hypnic jerk or sleep start.  Despite the name, it's not a hip new dance trend.  It's an involuntary twitch.  These jerks are basically just muscle contractions, kind of like a hiccup, and they're perfectly normal, most of the time.  Neuroscientists don't really know what causes them, but they could happen because of the way your body switches over to being asleep.

When you're awake, you're getting lots of signals from the part of your brainstem known as the reticular activating system, but come bedtime, a cluster of cells deep in the middle of the brain starts to block these signals and bring on sleep.  While this set-up makes for pretty good transition to dreamland, it's not entirely seamless, so hypnic jerks could be glitches that arise during this process.

The twitches might be related to some aspect of your hypnagogic hallucinations and they could wake you up, but most of the time, you don't notice a thing.

So while this not quite awake/not quite asleep state can be pretty freaky, usually, it just passes you by, which is probably good, because you have to go through it every single night and not everyone wants to be as surrealist as Dali.

Thanks for watching this episode of SciShow.  If you wanna learn more about weird brain things, we have a whole channel called SciShow: Psychology over at YouTube.com/SciShowPsych and for more of all kinds of science, you can keep watching right here or go to YouTube.com/SciShow and subscribe.