Amy K. Hooper
Posted: January 10, 2013, 1:55 p.m. EST
Ferret Dexter/Courtesy Kate & Trevor Brown
Ferrets have always been known to be sound sleepers.
Ferrets spend a lot of time asleep. That becomes pretty clear after spending any time around them. Matt Zail said his ferrets, Loki and Phoenix, spent 18 to 22 hours a day with their eyes closed. Other ferret owners report 14 to 20 hours a day of slumber.
“Mid-step while running down a hallway, they could fall asleep,” Zail said. “They could instantly become a speedbump. We were kinda prepared for [the behavior], but it’s not what you expect. It was more funny than worrying.”
Other times, Zail and his wife felt amazed at how soundly the ferrets slept. “They are such heavy sleepers,” he said. ”They’re all floppy in your hands. You can’t even tell if they’re breathing. It’s like deep hibernation.”
That tendency is known to many ferret owners as Sleeping Not Dead or SND; Dead Ferret Syndrome or DFS; and “dead sleep.” In fact, Bethanie Wright said her ferrets “sleep as heavily as though they’re dead.”
Behind Closed Eyes
Even though the ferrets act limp and lifeless, we know they’re still warm and breathing. What causes this unsettling behavior?
First, let’s look at sleep, defined by the fourth edition of Webster’s New World College Dictionary as “a natural, regularly recurring condition of rest for the body and mind, during which the eyes are usually closed and there is little or no conscious thought or voluntary movement, but there is intermittent dreaming.” The dictionary defines “dream” this way: a sequence of sensations, images, thoughts, etc. passing through a sleeping person’s mind.
Are humans the only creatures to dream? Do ferrets dream? How can we know that?
Consider this: Researchers have found that human dreams occur during REM sleep — “the periodic, rapid, jerky movement of the eyeballs under closed lids during stage of sleep associated with dreaming” (WNWCD).
Multiple studies have shown that ferrets experience REM. In fact, a study published in the February 1996 issue of “Sleep” carries the title “A preliminary study of sleep in the ferret, Mustela putorius furo: a carnivore with an extremely high proportion of REM sleep.”
In that 1996 article, the researchers report that ferrets in laboratory conditions spent 40 percent of their total sleep time in REM sleep. Considered a high amount of REM sleep, that percentage stems from multiple REM episodes rather than long episodes.
Ten years later, a study abstract in Volume 172, Issue 1 of “Behavioural Brain Research” stated, “Like other placental mammals, ferrets exhibited the main vigilance states of wakefulness, rapid-eye movement (REM) sleep and non-REM (NREM) sleep. Interestingly, the amount of REM sleep in the ferret was considerably higher than typically reported in placental mammals.” (Placental mammals include humans, dogs, cats, bats, rodents and whales.)
REM And SND
So, science has shown that ferrets experience REM sleep — and a lot of it. How does that relate to SND? REM sleep typically involves dreams because of an increase in brain activity – as well as paralysis in major muscle groups, states WebMD. That latter condition, muscle atony, might explain a sleeping ferret’s lack of response when his human tries to wake him up.
Unfortunately, no readily available studies appear to examine that possibility. Laurie Hess, DVM, DABVP, of Veterinary Center for Birds & Exotics in Bedford Hills, New York, concurred. “I am not sure that there have been studies linking the muscle atonia in ferrets in REM sleep to the ‘sleeping but not dead behavior’ we see in them, but it surely seems likely, as they do become quite limp at times when they are sleeping,” she said. “Given the large proportion of time scientists seem to say they spend in REM sleep, it is quite possible that this atony is occurring during REM sleep.”
Scientists haven’t shown that ferrets dream, but some veterinarians think they do. Hess isn’t the only one to say she thinks that these critters experience sensations and images while asleep. Mark E. Burgess, DVM, of Southwest Animal Hospital in Beaverton, Oregon, said personal observations lead him to think that ferrets seem to dream.
“They do occasionally twitch and move while sleeping, as if dreaming, though that's not proof positive … but they do tend to sleep soundly and very deeply, and with good muscle relaxation, so outward movements are often minimal,” Burgess said. “Without EEG readings, it would be hard to be sure of the dream state that they're in.”
Will we ever know if ferrets dream or if REM causes their utterly limp posture while asleep? Science might not answer those questions, but we do know that most ferrets tend to have two settings: Off and High.
Zail’s experiences with Loki and Phoenix tend to be universal: “They’re go-go-go when they’re active, and then they crash.”
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