sleepwalking pain perception
Most sleepwalkers do not feel pain when injured during these episodesKit/Flickr

Sleepwalkers do not appear to feel pain when injured during episodes of somnambulism, researchers have found. In a study of sleepwalkers with at least one episode that involved an injury, 79% felt no pain during the incident and remained asleep despite being hurt.

The team of scientists from the University of Montpellier and the Hopital Gui-de-Chauliac in Montpellier, France, found an intriguing paradox among the sleepwalkers. While they are unlikely to feel pain during sleepwalking episodes, they are at an increased risk of headaches and migraines when awake.

Publishing their findings in the journal Sleep, they wrote: "Our results highlight the clinical enigma of pain in sleepwalking patients with complaints of frequent chronic pain, migraine, and headache during wakefulness but who report retrospectively experience of analgesia during severe parasomnia episodes, suggesting a relationship between dissociated brain activity and nociceptive dysregulation."

In effect, the scientists say they think there is a link between sleepwalking and an impairment of the sensory neuron that responds to pain by sending signals to the spinal cord and brain. "Our most surprising result was the lack of pain perception during the sleepwalking episodes," said principal investigator Dr Regis Lopez. "We report here, for the first time, an analgesia phenomenon associated with sleepwalking."

Sleepwalking affects around 4% of adults, according to the American Academy of Sleep Medicine. It occurs when people are waking up from deep sleep, but the process is incomplete. The team looked at 45 men and 55 women diagnosed with sleepwalking, with an average age of 30. From this, 47 had reported injury during a sleepwalking episode. Ten reported waking up immediately because of the pain, but the other 37 felt nothing and only felt it when they woke later.

One patient broke his leg when he climbed on to the roof of his house and fell off, but he did not wake up until the morning. Another suffered severe fractures after jumping from a third-storey window. Again, they did not notice the injury until waking up later in the night.

"Our results may help to understand the mechanisms of the sleepwalking episodes," said Lopez. "We hypothesise that a dissociate state of arousal may modify the components of sleep-wake behaviour, consciousness, and also pain perception."