“Jack… Jack? Jack…!” A flash of lights and the whizz of cars speeding past suddenly assault my senses. In the background, I’m dimly aware of my name being called. In a few more seconds I have woken more fully and, as if suddenly coming up from underwater, I hear the shouts and then realise why my name being called out in increasingly frantic tones. “Close the door! Jack…! Close it!”
The car door was halfway open and I had just taken off my seat-belt. I was, as I began to realise, a mere second or two from stepping out of the family car hurtling along the M6. Suffice to say, I wasn’t trusted with the privilege of a child-lock-free place in the car after that.
Sleepwalking can be considered to be one of a number of different but related disorders which come under the global title of ‘disorders of arousal’. Confusional arousal, sleep-related eating disorder and sleep-related abnormal sexual behaviours are all considered to be forms of, or to be related to, sleepwalking. These are behaviours of differing complexity, all of which are performed while an individual is asleep and of which they typically have little to no memory afterwards. Sleepwalking is typically classified by seemingly automated behaviours which may appear to serve some purpose.
It is not uncommon in childhood and, unlike my own experience, may cause a little distress and embarrassment but hardly have the potential for injury or death. In fact, sleepwalking during childhood occurs in as many as 17% of children at its peak age of 11 years old, and it is generally accepted that we grow out of this behaviour. Indeed, only around 1-4% of the adult population report sleepwalking episodes. Given that the brain and our sleep and bodily rhythms go through major changes during adolescence, this shift from a common phenomenon to relative rarity is far from surprising. Incidentally, as we will later see, the changes in the structure of sleep from childhood to adulthood may explain why we may remember the odd embarrassing sleepwalking episode as a child but not as an adult.
However, it is considered less than benign if sleepwalking becomes common in our adult lives. In both childhood and adult sleepwalkers, there is a relationship between sleepwalking and anxiety, psychosis and other parasomnias, such as night terrors. The main concern in adulthood is the risk of physical harm to the sleepwalker or others. All of us have probably been exposed to what sleepwalking is and what forms it can take. This can range from simply walking around the house to preparing food, having a conversation or even, in rare instances, driving. Given the range of complex behaviours which can occur during a period of sleepwalking, it should come as no surprise that some consider its occurrence in adults far from benign. Instances of murder, sexual assault and personal injury are purported to have taken place during a sleepwalking episode.
It should be stressed that this is far from the norm among adult sleepwalkers. These concerns are rare and although the sleepwalker may harm themselves or their bed-partner, it is typically the loss of sleep and subsequent daytime sleepiness which are of primary concern to doctors.
What Causes Sleepwalking?
So, what actually causes sleepwalking and why do some people seem to be prone to it whereas others don’t? First we need to have an idea of what happens as we fall asleep and what our brain is doing as we drift off.
The brain does not go to sleep in a unitary manner. Although we may feel ourselves start to drift into sleep as we steadily lose consciousness, the brain enters a sleep state in a progressive, region-specific manner. That is, brain regions which have been active over the course of the day or prior to sleep take longer to enter a sleep-like rhythm. As a result of this facet of sleep physiology, it is possible to observe certain regions of the brain as awake while others are technically asleep. Among those prone to sleepwalking, it is observed that motor regions of the brain may exhibit ‘wakefulness’ while areas responsible for higher cortical functions such as the prefrontal cortex are ‘asleep’. This may explain why someone can seemingly carry out complex actions but yet still be asleep, but it does not tell us how this occurs in the first place. Why is it that this pattern of localised wakefulness only results in sleepwalking in a small subset of adults and a larger subset of children? It seems the pattern of sleep in sleepwalkers sheds some light on this question.
During a typical night’s sleep, we progress through cycles which see us move from light sleep to deep sleep and back again about 4-5 times. Each of these cycles lasts about 90-110 minutes and marks our shift from non-rapid eye-movement (NREM) sleep to rapid eye-movement (REM) sleep. NREM sleep consists of 4 separable stages and dominates the first half of the night, whereas REM sleep is where we typically dream and dominates the latter half of the night. These stages can be tracked using a technique called polysomnography, whereby electrodes are placed on an individual’s head and the underlying neural activity is recorded while they sleep.
Researchers are interested in the function of these different stages of sleep (NREM is made up of four distinct stages) and how they might serve memory consolidation, daytime functioning and even mental health. How does sleepwalking fit into all of this then? Well, research seems to point to the deepest stage of NREM sleep, known as slow wave sleep (SWS), as a potential culprit.
When you compare individuals with a history of sleepwalking against those without in the sleep laboratory, you find some interesting differences. Namely, those with the history of sleepwalking episodes tend to show a fragmentation of their slow-wave sleep compared to the controls. It seems that sleepwalkers are more prone to waking up, albeit very briefly, during SWS sleep, and it is this occurrence which may precede a sleepwalking episode. Moreover, sleep deprivation over 24 hours and experimentally induced awakenings during NREM sleep can induce sleepwalking. However, there’s an important caveat to this: these findings are only found in those already prone to sleepwalking episodes (i.e. a recorded history of adult sleepwalking). Individuals who did not report sleepwalking did not start sleepwalking purely because they were deprived of sleep, or woken up at just the right time.
Problems during SWS fits with our earlier identification of sleepwalking as being commonly found in childhood. The percentage of the sleep cycle taken up by SWS is increased in childhood and decreases as we grow older. As we have already identified, sleepwalking can occur during any stage of NREM sleep but it is during SWS that it is most commonly ‘found’. The next question we’re presented with then is what might lead an individual to be prone to these ‘micro-arousals’ during SWS? The short answer is that we’re still not quite sure. The evidence available suggests that genetic factors play an important role in leading someone to be prone to sleepwalking episodes. For example, you are more likely to have a disorder of arousal such as sleepwalking if another relative also suffers from it.
Other factors beyond genetics have been identified as being linked to sleepwalking. A review last year identified sleep deprivation (which we have covered), stress, forced arousal during sleep (such as needing to go the toilet or a loud noise nearby), an unfamiliar sleep environment and medications (such as those used to treat schizophrenia) as potential triggers for an episode of sleepwalking and other arousal disorders. Interestingly, what all of these factors seem to have in common is that they influence SWS –by either deepening or fragmenting this stage of sleep. This seems to fit with the neural activity characterisation of sleepwalking. Other research groups have identified alcohol as a trigger for sleepwalking, but the evidence is less convincing here. It should be noted that the above factors, bar sleep deprivation, have not been studied in any detail in the sleep laboratory. Anecdote does not science equal but, hopefully, it will provide a springy launch-pad for future research.
So, returning to the beginning and my story of my own sleepwalking episodes as a child. Sleepwalking, like sleep itself, is a puzzling but fascinating phenomenon. It may be that children are more likely to experience sleepwalking briefly as a facet of normal sleep architecture development. This likely explains my own, admittedly hair-raising, experience. However, a definite answer for what may cause sleepwalking in adults is less certain. Further research of known triggers and improvements in eliciting somnambulism in the sleep lab will help clarify these issues. Regardless, we already have some useful clues and it is likely that slow wave sleep problems will lead us in the right direction.
No doubt sleepwalking will remain of interest to scientists, playwrights, artists and anyone who has had even a brief brush with embarrassing or benign breaks in sleep. FYI, the story I decided to share with you is the least embarrassing sleepwalking episode I could think of.