Here’s my column at BBC Future from last week. You can see the original here. The full listof my columns is here and there is now a RSS feed, should you need it
As we give up our bodies to sleep, sudden twitches escape our brains, causing our arms and legs to jerk. Some people are startled by them, others are embarrassed. Me, I am fascinated by these twitches, known as hypnic jerks. Nobody knows for sure what causes them, but to me they represent the side effects of a hidden battle for control in the brain that happens each night on the cusp between wakefulness and dreams.
Normally we are paralysed while we sleep. Even during the most vivid dreams our muscles stay relaxed and still, showing little sign of our internal excitement. Events in the outside world usually get ignored: not that I’d recommend doing this but experiments have shown that even if you sleep with your eyes taped open and someone flashes a light at you it is unlikely that it will affect your dreams.
But the door between the dreamer and the outside world is not completely closed. Two kinds of movements escape the dreaming brain, and they each have a different story to tell.
Brain battle
The most common movements we make while asleep are rapid eye-movements. When we dream, our eyes move according to what we are dreaming about. If, for example, we dream we are watching a game of tennis our eyes will move from left to right with each volley. These movements generated in the dream world escape from normal sleep paralysis and leak into the real world. Seeing a sleeping persons’ eyes move is the strongest sign that they are dreaming.
Hypnic jerks aren’t like this. They are most common in children, when our dreams are most simple and they do not reflect what is happening in the dream world – if you dream of riding a bike you do not move your legs in circles. Instead, hypnic jerks seem to be a sign that the motor system can still exert some control over the body as sleep paralysis begins to take over. Rather than having a single “sleep-wake” switch in the brain for controlling our sleep (i.e. ON at night, OFF during the day), we have two opposing systems balanced against each other that go through a daily dance, where each has to wrest control from the other.
Deep in the brain, below the cortex (the most evolved part of the human brain) lies one of them: a network of nerve cells called the reticular activating system. This is nestled among the parts of the brain that govern basic physiological processes, such as breathing. When the reticular activating system is in full force we feel alert and restless – that is, we are awak
Opposing this system is the ventrolateral preoptic nucleus: ‘ventrolateral’ means it is on the underside and towards the edge in the brain, ‘preoptic’ means it is just before the point where the nerves from the eyes cross. We call it the VLPO. The VLPO drives sleepiness, and its location near the optic nerve is presumably so that it can collect information about the beginning and end of daylight hours, and so influence our sleep cycles. As the mind gives in to its normal task of interpreting the external world, and starts to generate its own entertainment, the struggle between the reticular activating system and VLPO tilts in favour of the latter. Sleep paralysis sets in.
What happens next is not fully clear, but it seems that part of the story is that the struggle for control of the motor system is not quite over yet. Few battles are won completely in a single moment. As sleep paralysis sets in remaining daytime energy kindles and bursts out in seemingly random movements. In other words, hypnic jerks are the last gasps of normal daytime motor control.
Dream triggers
Some people report that hypnic jerks happen as they dream they are falling or tripping up. This is an example of the rare phenomenon known as dream incorporation, where something external, such as an alarm clock, is built into your dreams. When this does happen, it illustrates our mind’s amazing capacity to generate plausible stories. In dreams, the planning and foresight areas of the brain are suppressed, allowing the mind to react creatively to wherever it wanders – much like a jazz improviser responds to fellow musicians to inspire what they play.
As hypnic jerks escape during the struggle between wake and sleep, the mind is undergoing its own transition. In the waking world we must make sense of external events. In dreams the mind tries to make sense of its own activity, resulting in dreams. Whilst a veil is drawn over most of the external world as we fall asleep, hypnic jerks are obviously close enough to home – being movements of our own bodies – to attract the attention of sleeping consciousness. Along with the hallucinated night-time world they get incorporated into our dreams.
So there is a pleasing symmetry between the two kinds of movements we make when asleep. Rapid eye movements are the traces of dreams that can be seen in the waking world. Hypnic jerks seem to be the traces of waking life that intrude on the dream world.
Mind Hacks 
Is it clear if the jerk causes the dream or if the dream startles and causes the jerk? The latter is how it feels to me, but the article seems to be implying the former. If it’s the former, subjectively the order of events seems wrong. Is it one of those cases where our perception of the order of the experiences is wrong?
Like Jon, I’d like to know about the order, too. I’ve only ever experienced hypnic jerks (that I recall) as part of a dream, and it really does *feel* as if it’s the result, not the cause.
It would be interesting to know how the “re-ordering” of events happens then.
To Jon and Victoria – according to the book The Head Trip: Adventures on the Wheel of Consciousness, it is as Tom said in his comment. Page 31: “Almost everyone seems to get [myoclonic jerks], and they are usually paired with an internal narrative of tumbling down stairs or slipping on ice. Oliver Sacks is particularly impressed with these jerks and he believes they showcase the brain’s amazing high-speed improvisational abilities. The accompanying mini-dreams seem to start before the jerk, but in reality, says Sacks, they must be stimulated by an original ‘preconscious perception’ of the incoming twitch, and the whole mini-story is built on the fly in less than a second.” By the way, this book (authored by Jeff Warren) is an amazing read. I highly recommend it.
Another interesting article, Tom, thanks.
My partner suffered from these hypnic jerks: a little while after going to bed, she’d suddenly kick a leg, or something, and scare the crap out of me. Weirdly, I’d never been conscious of doing it myself, but sometimes I’d jerk shortly after, just as I was returning to a restful state, as if in response. What’s going on there?
@Jon – yes, that’s exactly what i’m suggesting. We perceive
Dream fall off kerb -> jerk
But I’m suggesting
Jerk -> dream fall off kerb
@Danbettles. Possibly your wife’s jerk either raised your awareness enough to notice your own jerking, or it woke you up enough to allow your motor system to create a jerk.
I once did experiments with out-of-the-body experiences – I got to the stage where I was able to get up and walk off, leaving my body asleep. Ok. But remembering how I was in that state was very much ‘the planning and foresight areas of the brain are suppressed,’. One’s sphere of awareness is so limited, and one is barely capable of volition or independent action.
I became completely disillusioned with the whole thing. It is no more than very lucid dreaming, for which the dreamer learns certain procedures of interpretation, and promptings.
As far as ordering goes, I think the jerk would come first and then the brain would interpret it. Just as in the split brain experiments where you do something and give a reason later.
Dear Columnist,people are more simular and samekimd,people have I too that nice Term Hypnotic Jerk,I mostly nightmares.Deine Scandinavian lookinclass.
Recently I jumped out of bed from horizontal sleep to vertical standing. From what i remember i was under the impression that insects were crawling all over my walls. The thoughts going through my head was ‘what the fuck ahh’, but take into mind i’m short sighted and can’t see me arse from my elbow without my glasses, there was no chance i could have seen anything. I put it down to exam stress and realising nothing was there tried to get back to sleep.
Dear
iSociety,In dreams we can fly, space arounding me.Horror,vampireshouse is near.I skreem and woke up.My blood is running blueh.I was happy real wod,realize understudied.Yeats and so on.Yes i notice my imaginations.Deine Society,t(hank God)Freuline Friend.
Dear
iSociety,In dreams we can fly, space arounding me.Horror,vampireshouse is near.I skreem and woke up.My blood is running blueh.I was happy real wod,realize understudied.Yeats and so on.Yes i notice my imaginations.Deine Societyt(thank God)Freuline Friend.
I have experienced these hypnic jerks for many years now (much to the dismay of the other half), and I also suffer from what has been coined “exploding head syndrome”. Personally i’ve noticed the hypnic jerks are more prevalent when I am stressed or anxious, whereas the EHS only occurs when I am sleep deprived. Sunday gone I was actually suffering from both which was quite a pain, as it also increased my already elevated anxiety, due to lack of sleep. To me the hypnic jerk wakes me and I can’t recall interaction between it and my dream. I always thought it was a random physiological response possibly having something to do with the central nervous system misfiring. I wonder if there is any correlation between hypnic jerks and restless legs syndrome, above what you would normally expect from symptoms of sleep deprivation, the latter being an awake disorder…
Narcoleptic here. Finally, something I have knowledge on at an intimate level, but of course, I wish I didn’t.
With narcolepsy these can happen while mostly awake and going about one’s day. The meds that promote daytime wakefulness- Xyrem at night, stimulants by day- help reduce their incidence. Before we knew I had narcolepsy, I spent years trying to have my daytime “twitches” figured out.
They ruled out everything they could think of. When the narcolepsy was diagnosed, we all did a bit of a double-take, and reviewed the history of the twitches and spasms. Sure enough, they followed with the worst of the sleepiness. Solved! But no one would ever have thought to look for it, not even me.
So it’s possible for these to happen by day, and to spend years ruling out MS and everything else, only to find out that they are ultimately, “just one of those things,” that can go with a too-permeable sleep-wake barrier.
I’d like to know if people who experience hypnic jerks are more likely to talk in their sleep and/or sleep walk. Have there been any studies on this? Do you have any thoughts about it?
I’d like to know if people who experience hypnic jerks are more likely to sleep walk and/or sleep talk. Are there any studies on this? Do you have any thoughts about it?
Somewhat on topic…when I’m under moderate stress, I occasionally have a hypnogogic hallucination. It’s very weird. It usually happens in the wee morning hours where I hear the doorbell ring. It’s extremely vivid and is the same exact sound every time. It’s very disturbing because for a long time I thought someone was ringing my doorbell at 4:00 a.m. and running…repeatedly. I even called the police once, and they told me to set up a camera on a tripod to see who was doing it. Turns out, it was a hypnogogic hallucination. It now happens maybe once every 5 years.