Many sleepwalkers suffer an enigmatic existence. Their waking hours are plagued by pain that can dull their physical activity levels. Yet their lively nocturnal adventures can cause pain-free injury.

That’s the conclusion of a new study published in the journal Sleep by a group of French researchers. The team studied 100 patients who sleepwalk at least once a year but have no other sleep disorders. Compared to 100 non-sleepwalking participants, the sleepwalkers were more likely to suffer headaches, migraines, and chronic pain, as well as symptoms of depression and insomnia. But, of the 47 participants that reported being injured at one point during a sleep-like stupor, nearly 80 percent said the trauma was painless.

One participant had at one time fallen from a third-floor window while sleepwalking, sustaining severe fractures. Yet the participant slept through the fall and didn’t experience pain until being woken up. Another participant reported that he once climbed onto the roof of his house, fell off, broke his leg, and continued sleeping soundly until the morning. A third patient tumbled down the stairs while sleepwalking and slept in a jumble until his wife woke him. Once awake, it took several minutes before he noticed he was in pain, his wife reported.

The findings led the researchers to hypothesize that the oblivious state in which people sleepwalk disrupts parts of the brain responsible for pain processing as well as consciousness. They also hypothesize that there may be an underlying abnormal circuitry in the brains of sleepwalkers that explains both impaired pain perception—during waking and sleeping hours—and sleepwalking.

Previous research has found that people become less sensitive to pain as they fall deeper into sleep, Michele Terzaghi, a sleep researcher at the C. Mondino National Neurological Institute in Italy, told Ars, but this insensitivity may be exacerbated in sleepwalkers.

For pain to be sensed, there must be a connection between the circuits that send pain signals to the brain and the parts of the brain that decode those signals into pain. In sleepwalkers, that connection could be weakened, if not completely shut off, he said.

The hypothesis makes sense with what little scientists know about sleepwalking, Michael Howell, a neurologist at the University of Minnesota, told Ars. Sleepwalkers aren’t actually asleep as they ramble around at night. Sleepwalkers sleepwalk when their brain partially wakes up, but kind of gets stuck halfway. It’s actually a disorder of arousal, he said.

Previous research has found that with sleepwalking and arousal disorders, there’s localized activity in the motor cortex, the area of the brain responsible for movement. But the frontoparietal associative cortices, which are involved in conscious awareness and pain, show brain wave activity associated with deep sleep during episodes.

For now, researchers can only speculate on why sleepwalkers may experience more pain in their waking hours, though. Scientists know surprisingly little about sleepwalking, Howell added.

In the meantime, Terzaghi said, it’s important for clinicians to be aware that sleepwalkers may be at high risk of severe harm since they may not wake up while they’re being injured.

Sleep, 2015. DOI: 10.5665/sleep.5144 (About DOIs).