Imagine getting up in the middle of the night, and starting to drive your car around town or preparing a big dish of dog food and eating it—and although your eyes are wide open, you’re sound asleep. While these experiences sound like fiction, they are examples of a curious condition occurring around the edges of sleep, known as “sleepwalking.” Sleepwalking is a so-called “arousal disorder”—a form of partial awakening during sleep— that affects around 2.5% of adults and as many as 14% of children. And it’s this uncanny phenomenon that not only gives us profound insight into how the dissociative mechanisms during sleep come about and how they can collapse, but also provides clues about the nature of consciousness itself.
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Each night, you go through 90-minute cycles of sleep, moving through unique stages. During “light sleep” (known as Stages 1 and 2) your heart rate and body temperature drop, a sort of transition phase between wakefulness and sleep. But it’s in “deep sleep” (Stage 3) that your brain engages in critical housekeeping chores for your body. These include releasing hormones to repair your skin, replenishing your cardiovascular and immune system, and creating new memories. During this time, your neurons literally shrink allowing cerebral spinal fluid to bathe your brain and get rid of harmful toxins. Deep sleep is a form of restorative sleep, and oftentimes, it is difficult to awaken the sleeper. The fourth stage, the so-called rapid-eye-movement sleep or “REM” sleep for short, is called “paradoxical sleep.” And for good reason: your blood pressure, heartbeat, and breathing quicken, and your brain waves speed up. If we eavesdrop on the activity of neurons during REM, they resemble those of wakefulness. We have our most crisp, life-like, and emotional dreams during REM sleep. To prevent you from acting out these REM dreams and hurting yourself, your brain temporarily paralyzes your entire body. This transition is tightly controlled by chemicals released from the lower part of the brain that tilt you between sleep and wakefulness.
Read More: Why Sleep Paralysis Makes You See Ghosts
Sleepwalking occurs not during REM but deep sleep, dispelling the commonly held myth that the sleepwalker is a confused dreamer roaming around the house and acting out bizarre dreams. This is virtually never the case. Unsurprisingly, sleepwalking is more common in the early part of the night where deep sleep is abundant.
Unlike REM, the activity of the cortex during deep sleep—which sits on the outer layer of the brain, almost like a motorcycle helmet—is vastly slowed down. But this architecture of deep sleep can sometimes, inadvertently, break down: arousal from the brain’s emotional headquarters, the so-called “limbic system,” can become hyperactive and tickle the cortex resting above it, causing it to partially awaken the person from the deep slumber. Indeed, this arousal results in the brain being awake and asleep at the same time—as if the world of sleep and wakefulness were colliding.
Neural neighborhoods important for logical thinking and self-awareness like the dorsolateral prefrontal cortex (strips of cell tissue on the front sides of both brain hemispheres) remain napping while areas concerned with movement are widely awake. The former explains why the sleeper is so uncontrolled, confused, and lacks total insight; the latter why the person can move freely around. Emotional brain centers and the lower part of the brain essential for a sense of balance are also turned on. Interestingly, the memory region of the brain, a seahorse-like structure situated behind your ears, is shut off. This is why the sleeper can’t remember these sleepwalking expeditions.
While it is not known what exactly would trigger the brain to go into this odd sleep and awake state simultaneously, genes play a substantial role. In fact, a person can experiences sleepwalking 10 times more frequently if a close family member (a so-called first-degree relative) also experiences it. You’re also more than 5% as likely to experience it if your identical twin does (who shares 100% of your genes) versus your non-identical twin (who shares 50%).
Stress can also trigger sleepwalking, or at least make it more frequent and severe in those with these genes. Mental anguish is a major source of brain arousal which is difficult to switch off at night, known as nocturnal arousal. A striking example is those who’ve experienced trauma. A recent study showed that patients with PTSD have more and longer awakenings than healthy people do—they simply cannot sustain their sleep.
People who experience trauma are more likely to wake up during deep sleep because their overall sleep architecture is fragmented. Because of an aroused “emotional brain,” in particular an almond-like structure buried behind the ears called the amygdala, a person who has experienced trauma tends to pop in and out of sleep. The amygdala, which makes us feel trepidation and fear, is dancing with activity in those with trauma—and astonishingly, has often ballooned in size compared to those without trauma.
What’s more, people with trauma also have less and thinner nerve fibers, going from the front part of the cortex and their emotional brain that helps keep it in check, similarly to brakes on a car. Their emotional brain has virtually free reign, making their brain vulnerable to arousal during the deepest of sleep.
Uncontrollable arousal and the inability to sleep deeply also explains why this phenomenon is common in children in particular. In about 80% of cases. sleepwalking vanishes by the time children become teenagers. This is because the frontal cortex is not fully matured in children. It can simply not keep their nocturnal arousal in check which messes up their sleep depth.
The human brain is vastly more mysterious than what one could ever imagine, and sleepwalking (and the study of it) provides penetrating insights into consciousness. It shows us that what we call consciousness is not a single one-dimensional thing. Instead, it can sometimes represent distinct states, each with unique flavors. Yet other times, as this phenomenon elegantly illustrates, they can merge, producing, mixed brain states that blurs the crisp boundary between what it means to be asleep and awake.
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