I readily admit that I use to sleepwalk as a kid. My dad once found me laid out at the foot of our considerably large staircase completely unscathed! As I reflect back on those sometimes hazardous, but mostly humorous unconscious experiences I can't help but wonder if somnambulism, the formal term for sleepwalking, once served as some kind of adaptive function. Were our ancient ancestors afforded the opportunity to escape the perils of the wild during states of deep sleep?
There are countless stories of somnambulists executing complex escape behaviors, performing extraordinary feats, and seriously harming others. In 1987 Kenneth Parks drove 15 miles to his in-law's home, beat his father-in-law until he was unconscious, and stabbed his mother-in-law to death...doing all of this while asleep. He later went to the police station stating, "I think I have killed some people". He was covered in blood with a badly injured hand and had absolutely no recollection of what he had done. A year later he was acquitted of murder. Apparently one can get away with a lot on the basis of automatism; even the heinous crime of rape according to Ebrahim at the London Sleep Centre.
So why do people sleepwalk? Researchers like Pilon, Montplaisir, and Zadra have shown that probable causes of somnambulism can be due to a variety of factors such as stress, anxiety, sleep deprivation, or an external disturbance. Parks was unemployed, a gambaholic, and highly stressed. However, this doesn't really answer my original question. Was there an evolutionary purpose to this seemingly useless and bizarre phenomena or is it merely the behavioral manifestation of a sleep disorder?
Mahowald and Schenck at the Minnesota Regional Sleep Disorder Center seem to believe that somnambulism is a form of threat simulation gone awry during NREM sleep. This explanation suggests that there is, in part, an evolutionary role to the curious phenomena under consideration, but some kind of malfunctioning has taken place that doesn't serve any added purpose.
What if we were to look toward our primate brethren for the answer? Do apes and chimpanzees experience sleepwalking too? Kantha at Kyoto University conducted a literature review on somnambulism in non-human primates and found absolutely no evidence for this. However, she adds that it may be due to limitations in expertise and methodological resources.
For now it seems that somnambulism is found exclusively in humans and may have been a fairly recent phenomena set off by the ill's of modern man. I suppose if you're still curious about this fascinating subject you can conduct your own little personal experiment. The next time you witness a hunger repressed friend or loved one sleepwalking towards the fridge for a late night snack...try wrestling them to the ground and observe what happens.
Ebrahim IO (2006). Somnambulistic sexual behaviour (sexsomnia). Journal of clinical forensic medicine, 13 (4), 219-24 PMID: 16564199
Kantha SS (2003). Is somnambulism a distinct disorder of humans and not seen in non-human primates? Medical hypotheses, 61 (5-6), 517-8 PMID: 14592779
Mahowald MW, Schenck CH, Rosen GM, & Hurwitz TD (1992). The role of a sleep disorder center in evaluating sleep violence. Archives of neurology, 49 (6), 604-7 PMID: 1596195
Pilon M, Montplaisir J, & Zadra A (2008). Precipitating factors of somnambulism: impact of sleep deprivation and forced arousals. Neurology, 70 (24), 2284-90 PMID: 18463368