Sleepwalking, a phenomenon on the rise, is actually an intermediate state between sleep and wakefulness and can be dangerous to both one’s self and others. Considering neurophysiological findings, it may however, result from an adaptive preparation for escape.
Sleepwalking, formally known as somnambulism, is a form of parasomnia or arousal disorder, most common to children between the ages of four and twelve. It encompasses a range of active behaviour complex enough to be representative of a wakeful state, but with a lack of judgment and memory of the activity. In fact, there are some extremely remarkable cases of sleepwalking in adults that range all the way from harmless and amusing, to murderous. On the harmless side is an example of a woman who in her sleep, was “pissed off” because she can’t open a “tomato cage,” and a man who amazingly earns a significant amount of money from drawings he produces while asleep. Serious incidents include a teen who jumped out of a window 25 ft. above ground and another individual who froze to death in his sleep. Probably the most serious case is that of Kenneth Parks, who drove 23 km and then murdered his mother-in-law. Notably, sleepwalking as a criminal defense has become a growing topic in legal and ethical studies. This is largely because sleepwalking is a phenomenon on the rise, potentially as a side effect of increased use of sedatives, such as Ambien that have been argued to precondition parasomniac behaviour. Thankfully, however, most sleepwalking incidents are on the harmless side.
So what do we know about the cause of sleepwalking? Over millennia, sleepwalking was thought to result from everything from dryness of the brain, to vapors that stem from digestion to an imbalance in body humors. One common theory was that it was an enactment of dreams, most often occurring during REM sleep. Relatively recently however, Roger Broughton (1968) discovered that sleepwalking rises it’s ‘zombies’ out of Non-REM (NREM) sleep, specifically deep, slow wave sleep (SWS). Unlike REM sleep, which is characterized by low amplitude EEG waves varying in frequency, SWS is defined by high amplitude oscillations, low in frequency. These patterns are known as delta waves.
Sleepwalkers actually seem to have a subconscious eagerness to rise out of this deep sleep stage. Localized cortical areas that underlie motor activity are more likely become fully active, showing patterns of activity that are remarkably similar to those observed during a wakeful state. Meanwhile, frontal and parietal associative areas that interface one’s goals with incoming sensory information maintain or even increase their sleepy delta wave pattern. When the frontoparietal network is in such an inactive state, its usual role in inhibiting weird, non-goal directed behaviour becomes subdued. In fact, a direct, inhibitory link between frontoparieteal areas and the cingulate gyrus, an area central to complex motor activity, has been implicated as a prime sleepwalking root. It’s as if, during sleep, the motor area’s arousal threshold has been lowered compared to other parts of the brain. Why would that be?
Some think this motor-readiness was an adaptive feature that possibly helped survival. Sleeping, by definition is characterized by a loss of consciousness. However, this loss of consciousness is actually, surprisingly incomplete. In fact, using multiple imaging and recording techniques, it’s been demonstrated that auditory stimuli presented during sleep, can trigger activation in both the auditory cortex and the amygdala – a center directly related to the emotional ‘fight or flight’ response. What’s even more intriguing is that even while sleeping, our brains can selectively discriminate and process unfamiliar sounds (What made that noise??) and emotionally meaningful stimuli such as one’s own name. It seems we are tuned to process behaviourally relevant stimuli, even while we are fast asleep. This is probably not a coincidence, because novel and/or meaningful stimuli, could signal the presence of danger. Considering that we’ve had to avoid and escape potential predators for as long as we’ve been part of the animal kingdom, it’s been highly adaptive to evolve an arousal system that maximizes the benefits of sleep while minimizing the chance of being eaten. Potentially, sleepers without a low threshold for motor circuitry activation got eaten.