If a sleepwalker hits their pinky toe on a corner, does it hurt like hell? Do they wake up? Or does it not hurt all and they just keep walking? I must confess that this question hadn’t really crossed my mind until I came across an article titled “Pain in sleepwalking: a clinical enigma”. That sure spiked my curiosity.

Sleepwalking, formally known as somnambulism, is a type of parasomnia; this term refers to all the odd things that can happen during sleep, including abnormal movements, behaviors, emotions, perceptions, and dreams. Most parasomnias are dissociated sleep states in which partial arousal occurs during the transitions between wakefulness and sleep states.

Sleepwalking is a disorder characterized by the occurrence of motor behaviors during sleep. These behaviors can range from just sitting up to walking around the house, in the best case scenario. In extreme situations, sleepwalkers may actually leave the house and even drive long distances.

There’s a myth that states that sleepwalkers should not be woken up. The opposite is actually true since it can be dangerous not to wake a sleepwalker and just let them unconsciously roam about. The only problem in trying to awake a sleepwalker is that it sometimes induces violent attacks on the person trying to awaken them.

By interfering with the quality of sleep, somnambulism induces fatigue, daytime sleepiness, impaired vigilance in the morning, leading to psychological distress and poorer quality of life. In sleepwalking, only slow wave sleep (deep sleep) is disturbed, and it usually happens precisely during arousal from deep sleep.

Research suggests that sleepwalking is a dissociative state caused by disturbances in local sleep mechanisms due to incomplete arousal from deep sleep. Local sleep is a phenomenon wherein neurons in one particular region of the brain go “offline” while the other regions remain “online”; this means that that particular region is asleep, while the rest of the brain is awake. Local sleep is very common after long sleep deprivation periods.

But how does that affect pain perception?

Pain and sleep

There is a clear association between sleep and pain: on one hand, disturbed sleep is a major complaint of subjects suffering from chronic pain, and on the other hand, sleep deprivation is known to increase pain sensitivity. Pain occurring during sleep increases arousal frequency, thereby disrupting sleep.

In the case of somnambulism, it is hypothesized that some dissociated brain activity combined with dysregulated occurrence of local sleep may affect regions involved in pain perception. One the other hand, and although the reason is unclear, there is a high incidence of sleepwalking and sleep terrors in patients suffering from migraine. Chronic pain is also frequently reported in other sleep disorders such as insomnia or narcolepsy.

In the study mentioned above, the authors focused pain experienced by sleepwalkers. The study compared the frequency of chronic pain, headache, and migraine between sleepwalkers and controls, and examined the impact of pain in sleepwalkers. Pain perception was also retrospectively assessed for patients who had experienced injury during parasomnia episodes.

First off, their results showed that the proportion of sleepwalkers who suffered from chronic pain was surprisingly high, with headache, migraine, and chronic pain being significantly associated with somnambulism. Also, sleepwalkers reported more frequent daytime sleepiness, insomnia, and depressive symptoms.

Among the sleepwalkers who reported violent parasomnia episodes, almost 80% stated that they had perceived no pain during those violent episodes, having remained asleep despite serious injury, in some cases.

This article thus showed a duality in the sleep-pain connection: although sleepwalkers tend to report chronic or recurrent pain conditions during wakefulness, they experience analgesia during parasomnia episodes. This is an intriguing relationship and its mechanisms are still poorly understood.

Sleep and pain interact in intricate ways that are influenced by several biochemical and psychological factors. In somnambulism, the brain is partially awake, and partially in non-REM sleep, resulting in no conscious awareness of actions. Previous studies with sleepwalkers have found changes in neuronal networks that also play a role in pain regulation. It is possible that a dissociated arousal state in certain brain regions may affect sleep-wake transition states, along with consciousness and pain perception.

Studies in healthy volunteers have shown that painful input processing is diminished during sleep, preventing awakening in response to meaningless sensory inputs. But since even serious injury may not awake a sleepwalker, this process is most likely reinforced in this context.

However, as the authors of the study underline, since sleepwalking episodes are often associated with lack of conscious awareness and memory of the event, the hypothesis that sleepwalkers do feel pain but just can’t recall it cannot yet be discarded. Nevertheless, it seems clear that pain and sleep can deeply influence each other.

References

Lautenbacher S, Kundermann B, & Krieg JC (2006). Sleep deprivation and pain perception. Sleep medicine reviews, 10 (5), 357-69 PMID: 16386930

Lavigne GJ (2010). Effect of sleep restriction on pain perception: towards greater attention! Pain, 148 (1), 6-7 PMID: 19914777

Lopez R, Jaussent I, & Dauvilliers Y (2015). Pain in Sleepwalking: A Clinical Enigma. Sleep PMID: 25902807

Moldofsky H (2001). Sleep and pain. Sleep medicine reviews, 5 (5), 385-396 PMID: 12531004

Schrimpf M, Liegl G, Boeckle M, Leitner A, Geisler P, & Pieh C (2015). The effect of sleep deprivation on pain perception in healthy subjects: a meta-analysis. Sleep medicine, 16 (11), 1313-20 PMID: 26498229

Zadra A, Desautels A, Petit D, & Montplaisir J (2013). Somnambulism: clinical aspects and pathophysiological hypotheses. The Lancet. Neurology, 12 (3), 285-94 PMID: 23415568

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