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Swimming in her sleep? How the Idaho woman did it

You may have heard about the 31-year-old Idaho woman who last week woke up after “sleep swimming” in the Snake River. She was found, suffering from hypothermia, along the shore. It was the second time she’d done it this summer.

Sounds ridiculous, right? Sure, we’ve all heard of bizarre cases of “sleep driving” and “sleep eating” and even “sleep sex,” but sleep swimming? In a cold river? (I’ve been in the Snake River in Idaho, and I can tell you, it’s cold.) Surely, even if you could walk to the river in your sleep, that first splash of icy water would wake you.

Not necessarily. 

The problem with that popular notion, explained Dr. Mark Mahowald, visiting professor at Stanford University’s parasomnia clinic (parasomnia refers to sleep disorders involving behavior), is that “sleep” doesn’t have to be an all-or-nothing affair.

Parts of your brain can be “asleep” while other parts are “awake.” Brain regions, mainly the frontal cortex, responsible for self-control, reasoning and laying down memories can be happily snoozing while parts of the deep brain, like the stem, can be “awake.”

Because deep brain regions have been equipped with what Mahowald calls “central pattern generators,” patterns of physical acts, something like one of those dance steps instructors lay down on a floor, “you can have somebody who is sleepwalking, and be capable of performing extremely complex behaviors.”

He likens it to a chicken that goes under the hatchet and then starts running around, headless, through the barnyard. The chicken doesn’t have any brain at all, and yet, because its spinal cord has complex pattern generators in it, the chicken runs.

“In humans,” he explained, “there is good evidence that complex behaviors like running, screaming, shouting, sex acts, are all pre-packaged central pattern generators,” either learned or instinctive, that are let loose while the rational brain is sleeping.

You’d have to know how to swim in order to swim in your sleep, of course -- you can’t just start playing Chopin in your sleep if you don’t already know how -- but this explains why people can drive, sometimes long distances, and not recall ever doing it.

People have actually committed murder while sleeping, as documented on a website run by Mahowald and colleagues.

The cold water of a river wouldn’t necessarily wake the sleeper because sleep is like being in a state of anesthesia. We tend not to feel pain or discomfort while sleeping, only after the event is over.

“We had a case in Minnesota where a guy sleep walked outside, at 20 below zero, and sustained incredible frostbite on his feet,” Mahowald recalled. “He wasn’t aware until he woke up and saw the blisters.” 

The causes of sleep walking vary, but may have a genetic component, explained Dr. Steven Poceta, a neurologist affiliated with the Scripps Clinic Sleep Center in San Diego who has documented cases of sleep eating, driving, and cooking among other behaviors.

While science hasn’t figured out why many children sleep walk, but outgrow it, it does appear that the habit can return during times of stress. “That’s a very common scenario,” Poceta said.

Sleep deprivation caused by apnea, or restless leg syndrome, can lead to sleepwalking. So can drugs like zolpidem (Ambien), which became infamous for leading to sleep eating. As for how often that happens, Poceta said, “we think it is more common than is fully appreciated.”    

Brian Alexander ( is co-author, with Larry Young PhD., of "The Chemistry Between Us: Love, Sex and the Science of Attraction," ( to be published Sept. 13.