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Sleepwalking is relatively common — somewhere between two percent and four percent of adults rise during deep sleep and take unconscious constitutionals. Plenty of research has focused on behavioral tendencies of sleepwalkers before, during and after night-wandering episodes occur. But, we know far less about brain traits of sleepwalkers during daytime hours, when they’re awake and otherwise behaving normally.

A new study, published in PLoS One, has unearthed day-time brain abnormalities that distinguish sleepwalkers from non-roaming resters. These abnormalities, however, only appear to surface when people are sleep-deprived, further probing the critical, nuanced and still somewhat unclear relationship between sleep deprivation and sleepwalking.

Sleepwalkers, unsurprisingly, tend to report feeling drowsy during waking hours, presumably in part because moonlit sauntering isn’t terribly conducive to restorative rest. Some research depicts excessive daytime fatigue as a core clinical feature of the disorder. And it’s fairly well accepted that tiredness both stems from and feeds sleeping-walking episodes. In other words, all that slumber cardio leaves people tired, which, in turn, makes them more likely to get the shoddy sleep that predisposes them to waking up and walking around in the first place.

What’s not so clear about the vicious deprivation-walking cycle is what’s going on inside the brains of drowsy wanderers when they aren’t on-the-go. In the current study, researchers sought to flesh out this glossed-over neural process.

Ten sleepwalkers and 12 clinically ordinary sleepers participated in the study. Researchers performed brain scans using a neuroimaging device called Single Photon Emission Computed Tomography (SPECT), which has helped clarify waking brain differences in other sleeping disorders, such as narcolepsy. They scanned participants' brains on two separate mornings: once after a full night’s sleep and a second time after a night without shut-eye.

Researchers measured changes in cerebral blood flow in the inferior temporal cortex, a brain region that’s involved in processing visual information and, based on previous research, is particularly vulnerable to the effects of sleep deprivation. Also, sleepwalkers frequently hallucinate, indicating some type of glitch in this cognitive process.

Compared to disorder-free participants, sleepwalkers, the scans revealed, exhibited a noticeable blood-flow drop, but only after the night of deprivation.

What transpires during sleepwalking episodes varies immensely from person to person, and even between different episodes in the same person. Such variability, study authors wrote, translates to brain activity too — a consistent picture of the sleepwalking brain is hard to come by. As a result, it’s not necessarily that meaningful (or easy) to compare brain activity too close to episodes (temporally speaking). Researchers have had more luck fleshing out the neural mechanisms underlying other sleep disorders during windows of more predictable brain activity,  i.e., the day-time.

In this case, study authors believe their results highlight how sleep deprivation “not only facilitates the occurrence of sleepwalking episodes in predisposed patients but also reveals functional brain patterns during sleepwalkers’ wakefulness.” And there ya have it.