Does a platypus dream at night?

Note: This blog was originally published on the Helix Blog. At the moment, my html chops are only so-so. In order to see this blog in its full glory, I strongly recommend navigating to its original home here.

Are they cute, odd, interesting, or a combination of all three? However you describe it, there’s no escaping the fact that a platypus is a fascinating animal. It produces milk like a mammal but lays eggs like a reptile, and males have venomous prongs on their hind legs. Thanks in part to this odd mix of traits, the platypus has become an invaluable window into mammalian evolutionary history. It may even be able to teach us about the evolution of our dreams.

In essence, dreaming is a process where our brains ignore the world around us, focusing instead on visions, sounds, and emotions of their own making. It’s a powerful demonstration of what our minds are capable of, and yet we’re not sure why we dream. For some reason, we’ve evolved complex networks of neurons and biochemicals that all work together to make us sleepy, and put us into a deep sleep where dreaming occurs. To try and understand why we do this, scientists have turned to nature for possible answers.

Dreaming is known to take place most vividly during rapid eye movement (REM) sleep—a state of “deep” sleep where our bodies become temporarily paralyzed and our brains exhibit a unique signature of electrical activity [1]. For most people, it takes about 90 minutes to enter REM stage after falling asleep. During the intervening time, a person’s body will slowly relax and they may begin to form foggy, rudimentary dreams. This non-REM (NREM) sleep stage has its own identifiable electrical pattern that makes it possible for scientists to categorize the different stages of sleep a person is in [1].

These sleep stages aren’t uniquely human—REM-like sleep has been found in many different types of animals, including the platypus [1].

🤔What is the genetics behind nightmares?

Researchers have noted that some families tend to have nightmares more frequently than others—indicating a possible genetic basis to having nightmares [6]. Thus far, there’s very little research on the topic, however, research into sleeping disorders has shed some light.

It appears that people may inherit variants that cause a misalignment between our body’s state of sleep, and our brain’s state of sleep [7]. A good example of this is found in people with narcolepsy, who tend to enter and exit REM sleep stages faster than others. They also may experience a temporary paralysis in the body (like that experienced during REM) while their mind is fully awake. Notably, it’s common for people with narcolepsy to have an increased frequency of nightmares [5].

While indirect, observations like these suggest that some aspect of our dreams may be influenced by genetics—though we are a long way away from definitively connecting a person’s DNA sequences to their different types of dreams.

Researchers have measured the observable electrical patterns in the brains of sleeping platypuses and found that they too exhibit signs of both REM and NREM sleep [2]. Unlike humans, the platypus seems to have just one stage of sleep in which some electrical features of NREM sleep are present, as well as behavioral features—muscle paralysis, twitching, rapid eye movement—characteristic of REM sleep [2]. It’s also notable that the platypus spends more time in this REM-like sleep than any other studied mammal [3]. The observation that the platypus appears to sleep like we do, and experiences a phase of sleep associated with dreaming, suggests that this may be an important feature of our biology.

The platypus belongs to one of the oldest living groups of mammals, known as the monotremes [4]. By studying the platypus and other ancient animal groups, researchers can learn how mammalian traits evolved over time. The presence of REM-like sleep in the platypus indicates that this stage of sleep may be significantly important—so important that it has persisted through hundreds of millions of years of evolution. In fact, REM-like sleep has been found in a wide range of animals including ostriches, bearded dragons, emperor penguins, and many types of mammals [1]. The wide distribution of REM-like sleep in the animal kingdom further supports the conclusion that this type of sleep has an important biological role.

Did we inherit REM-like sleep from an ancient animal?

Research into sleep has given scientists only enough evidence to speculate about why sleep, and specifically REM sleep, is important. One line of thinking suggests that REM-like sleep gives our brains a chance to fine-tune our neural circuitry. Twitching is a common feature of REM sleep that some researchers believe may result from the brain testing its motor neurons [1]. Other lines of evidence indicate that REM sleep is a critical period in which our brains consolidate and solidify memories—though it’s been difficult for scientists to collect definitive evidence in support of this. What researchers have found is that the neural circuitry involved in REM sleep appears to be remarkably similar across animal species, meaning the platypus’s brain probably achieves a REM-like state in the same way we do [1].

As a widespread biological trait, REM-like sleep is possibly—if not likely—influenced by the DNA we inherit from our parents. We have seen that how long people sleep, when they go to sleep, and how well they sleep are all influenced by the DNA they inherit [5]. If REM sleep is the same way, it opens the door to further questions about the heritability of dreams. One can’t help but wonder if some ancient animal passed on the genes for dreaming, eventually allowing humans, birds, and even the platypus to explore their own dreamlands.

References

1. Peever, John, and Patrick M. Fuller. “The Biology of REM Sleep.” Current biology : CB 26.1 (2016): R34–R35. PMC. Web. 25 Sept. 2018.

2. Siegel, J.m., et al. “Sleep in the Platypus.” Neuroscience, vol. 91, no. 1, 1999, pp. 391–400., doi:10.1016/s0306-4522(98)00588-0.

3. Lesku, John A. et al. “Ostriches Sleep like Platypuses.” Ed. Evan Balaban. PLoS ONE 6.8 (2011): e23203. PMC. Web. 25 Sept. 2018.

4. Warren, Wesley C. et al. “Genome Analysis of the Platypus Reveals Unique Signatures of Evolution.” Nature 453.7192 (2008): 175–183. PMC. Web. 25 Sept. 2018.

5. Sehgal, Amita, and Emmanuel Mignot. “Genetics of Sleep and Sleep Disorders.” Cell 146.2 (2011): 194–207. PMC. Web. 10 Sept. 2018.

6. Hublin, Christer, et al. “Parasomnias: Co-Occurrence and Genetics.” Psychiatric Genetics, vol. 11, no. 2, 2001, pp. 65–70., doi:10.1097/00041444-200106000-00002.

7. Simor, Péter et al. “Disturbed Dreaming and the Instability of Sleep: Altered Nonrapid Eye Movement Sleep Microstructure in Individuals with Frequent Nightmares as Revealed by the Cyclic Alternating Pattern.” Sleep 36.3 (2013): 413–419. PMC. Web. 24 Sept. 2018.