Now how cool is THAT!!!! :0)
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 awake.
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.
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.
