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Opossum playing possum

Tonic immobility, or “playing dead”, is a defense mechanism found throughout the animal kingdom. It is a prey animal’s last line of defense against a predator after attempts to fight or to flee have been unsuccessful.  In a paper published in the December 2012 issue of Dreaming, the journal of the Association for the Study of Dreams, Ioannis Tsoukalas offers a hypothesis that REM sleep, found in all mammals, evolved from tonic immobility.

When an animal experiences tonic immobility, it is becomes immobile and appears to be dead. Animal researchers think that this hinders the predator from giving the final death blow or bite, giving the captured prey one final chance to escape.

There are a number of features common to both tonic immobility and REM sleep.

The most apparent is, of course, decreased muscle tone and almost complete paralysis. In both cases, reflexes are suppressed.

The back and forth eye movements from which Rapid Eye Movement sleep gets its name are also found in captured prey animals, who follow the gazes of their captors closely in order to predict their captors’ next movements. Scientists can even cause prey animals in a laboratory to exhibit these eye movements, using plastic eyes with no head or body attached. The ability to know what your captor is going to do next can mean the difference between living and dying.

Heart and respiration rate increase during REM sleep, as they do during tonic immobility.

Similarities exist between brain activity during tonic immobility and during REM sleep.

During both, EEG patterns resemble those of waking.

The locus coeruleus, a small section of the brain stem triggers the fight or flight reflex and plays a role in triggering REM sleep.

Acetylcholine and noradrenaline, neurotransmitters with an important role in regulating the sleep-wake cycle, also help an animal perform the actions it needs to make a successful escape.

Another neurotransmitter associated with the regulation of the sleep-wake cycle is oxytocin, which released during tonic immobility, along with opiates, as means of helping injuries to heal.  There is evidence that REM sleep is conducive to emotional healing.

Melatonin, also involved with sleep-wake cycle regulation –some people even use it as a cure for insomnia – is associated with tonic immobility. Scientists can induce tonic immobility by injecting melatonin in some animal species, and melatonin production can increase when these animals are exposed to stress or aggression.  Animals are more susceptible to tonic immobility at night, when large predators are frequently lurking about, seeking victims.

Studies of dream content also reveal an association between dreaming and negative emotions. Most dreams have negative emotional content.  This could come from the fact that dreaming evolved out of a response to the most negative situation imaginable.

Tsoukalas notes that species whose young take longer to develop and be independent tend to have more REM sleep. Human children, for example, have very high levels of REM sleep. Baby opossums also take a long time to become independent, and they too, experience a large amount of REM sleep.

Opossums are, of course, also well known for displaying tonic immobility, sometimes described as “playing possum”. Immobility is associated with youth in mammals.  Carrying a puppy or a kitten by the neck will immobilize it. This makes it easier to move the puppy or kitten without injuring it.  During REM sleep, paralysis is most apparent in neck muscles.

We can see the relationship between REM sleep and tonic immobility most clearly in cases of narcolepsy, a sleep disorder characterized by sleep paralysis, hypnagogic hallucinations and cataplexy.

During sleep paralysis, which occurs during the transition from sleeping to waking or form waking to sleeping, the victim is aware of his or her surroundings but is unable to move.  Sleep paralysis, which people who do not have narcolepsy may also experience, is often associated with a feeling of being trapped, frightening hallucinations and the feeling of being observed by a terrifying presence.

Hypnagogic hallucinations, which may also be experienced by non-narcoleptics, are often very frightening. They usually occur when trying to fall asleep or upon waking up.

Cataplexy is a condition when the patient, though awake, is overcome by severe muscle weakness and may collapse. In people with narcolepsy, it is usually triggered by intense emotion.