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Exploring the Mystery of REM Sleep

Did REM sleep evolve out of a primitive defense mechanism?

“To sleep, perchance to dream”

What role does REM sleep play in humans and animals? And why do we dream at all? Ever since the 1950s, researchers have known that sleep occurs in different stages and that the deepest part of sleep, which usually happens in the early hours of the morning, can be recognized by rapid, random eye movements.

This stage of sleep is now known as REM (rapid eye movement) sleep and is linked to extremely vivid dreaming. Among the different theories about REM sleep’s function are that it helps in forming new memories, stimulates the central nervous system, and restores brain chemistry to a normal balance.

Along with rapid eye movements, REM sleep is also recognized by low muscle tone and a rapid, shallow EEG pattern. In most adults, REM sleep can happen four or five times on average each night making up only 25% of the total time spent sleeping. Researchers have also found evidence of REM sleep in almost all land-based mammal and bird species and even some aquatic mammals such as dolphins.

As for how REM sleep first evolved, a recent article published in the journal Dreaming has suggested an intriguing new hypothesis. Written by Ionnanis Tsoukalas of Sweden’s Stockholm University, the article suggests that REM sleep, and dreaming in general, evolved out of an ancient defense mechanism still seen in many species. This defense mechanism is known as tonic immobility, or the faint response.

When rabbits react to apparent threats, they suddenly become rigid and unmoving. In fact, they can often seem completely dead which is usually their last line of defense when “fight or flight” is no longer an option. By “playing possum”, a rabbit can avoid a fatal bite by blending in with the surroundings and possibly escape.

Tonic immobility is seen in a wide range of land-based and aquatic species, including sharks, lizards, and many small mammals and birds. Researchers have managed to trigger tonic immobility using electric shock, or even gentle stroking in some cases. Reports of “animal hypnosisdating back to the 17th century have been based on the tonic immobility responses in animals such as rabbits and chickens. Though some researchers have suggested that it also can occur in humans experiencing extreme trauma, this remains controversial.

According to Tsoukalas’ hypothesis, REM sleep shares many common features with tonic immobility, including inability to move while sleeping, also known as sleep paralysis. As he points out in his article, “A fearful stimulus can induce the fight-or-flight reflex—which increases the muscle tone of the extremities—but it can also induce tonic immobility—which results in a gradual or sudden decrease in muscle”. This leads to what he refers to as a “fight-flight-or faint reflex”.

Among other ways that REM sleep and tonic immobility are similar are:

  • Both states show a similar EEG pattern with additional “theta” waves linked to the brain’s hippocampus (which plays an important role in memory and spatial perception)
  • Both states show strong loss of muscle tone and suppression of reflexes
  • Both states show changes in heart and breathing rate
  • Both states affect the body’s ability to regulate body heat
  • Both states show changes in the brain’s biochemistry including increased acetycholine and decreased serotonin and norepinephrine in the brain stem
  • Significant changes in eye movement, facial twitches, and jerking limbs in both states
  • Also, both states rarely last longer than a few minutes, varying from five seconds to 20 minutes

Both tonic immobility and REM sleep appear to be linked to parts of the brainstem involved with sleeping, eating, and breathing. They also appear to have very similar roles in helping the body recover from traumatic experiences. As an emergency reaction to extreme danger, tonic immobility prepares the body to cope with trauma and heal afterward. This happens through the release of brain chemicals such as acetycholine to reduce pain and increase drowsiness. Research into REM sleep has found many of these same biochemical markers which suggests that the vivid dreaming experienced during REM episodes plays a strong role in helping the body cope with stress and trauma.

One of the most intriguing suggestions that Tsoukalas raised was that the link between tonic immobility and REM sleep might provide evidence for the threat rehearsal theory proposed by evolutionary psychologists. Basically, the theory proposes that dreaming can be used to simulate threatening events to provide dreaming animals and humans with a way to review different ways to avoid these threats when awake. In the same way tonic immobility allows animals to avoid extreme threats, REM sleep may well let us practice how to cope successfully, as well as giving our bodies the chance to “recharge our batteries”. Even the rapid eye movements we show during dreaming can reflect the need to gather as much information as possible about our surroundings, much as we would if we were awake and in a dangerous situation.

Though Ionnanis Tsoukalas’s hypothesis raises fascinating questions about the origin and purpose of dreaming, more research is still needed to test the relationship between REM sleep and tonic immobility. Along with learning more about why we dream, new research can help us understand why our perception of reality is so distorted when we sleep and possibly to discover more about less-understood phenomena such as “near death experiences”, and “waking dreams”. As we learn more about dreaming, we might develop better tools for dealing with sleep disorders as well as better ways of helping the body heal after trauma.

Solving one of the fundamental mysteries about how and why REM sleep occurs may truly be such stuff as dreams are made of.

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