I magine 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.
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.