You may have more in common with your guppy than you realized. New findings published in Nature on Wednesday imply that the kind of sleep we experience evolved 450 million years ago, in the early stages of vertebrate development.
Zebrafish, we learn, experience similar sleep stages to other vertebrates, from reptiles to birds to mammals: Light intermittent sleep (during which we dream), and deep sleep, during which we usually do not dream. The inference is that fish undergo similar experiences.
Frankly, every cat or dog owner with eyes is confident the animal experiences dreams, and some of us have the pretense of interpreting them too (“Bowser is hunting chickens again in his sleep”). That’s a bit of a stretch given that, despite our verbal capabilities and the Viennese school of psychology, we can’t interpret human dreams — it doesn’t seem the ancients who thought dreams were messages from the gods had it right. In fact, science still isn’t sure about people who claim never to dream: maybe that’s true, maybe it isn’t.
But we can definitely objectively measure the hallmarks of human sleep using the physiological markers of two states: deep sleep, as opposed to rapid eye movement (REM) sleep.
Both states have been identified in the dog, cat and other mammals, in birds (which evolved from dinosaurs) and reptiles. Now is the first time the states have been identified in tiny little fish, whose evolution predated us all.
Tales of the heart
These sleep states in the zebrafish were associated with hallmark muscle signatures of our stages of sleep, including movement of the eye and heartbeat, says the team led by Philippe Mourrain of Stanford University.
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It makes some sense. Scales, feathers and hair had a common ancestor, so why shouldn’t our sleep?
So, it seems that the essentials of a good night’s sleep evolved before fish, reptiles, mammals and birds did (in that order). Perhaps our common ancestor some half-billion years ago — apparently a primeval proto-fish like Metaspriggina, which lived from 540 million years to 490 million years ago — could dream a dream.
Maybe it had nightmares too. Why not? Predators existed then too, though they wouldn’t have had jaws just yet. Metaspriggina didn’t. Jaws are believed to have developed only around 430 million years ago.
When zebrafish sleep
Zebrafish have become the standard among scientists wanting to check what fish do, because they are hardy little beasts and their genetics and physiology are thoroughly known. In this case, Mourrain and colleagues from three continents measured the brain-wide activity of sleeping 2-week-old baby zebrafish (aka larvae).
Result: The authors claim to have identified neuronal sleep signatures for the first time in fish. They identified not only deep sleep and REM but slow-bursting sleep.
Why do we need sleep so badly? Some have associated sleep with memory consolidation. But in May, a separate team of scientists, also working with zebrafish, discovered that when a baby zebrafish sleeps, so do the nerve cells in its brain. Which means the nerve cells “power down,” and instead of transmitting signals to and from the brain, they “rest.”
As the nerve cells take a break from their usual function, their resources are freed to reduce DNA damage that was accumulated during wakefulness.
Without sleep, the nerve-cell DNA is not properly repaired and the ultimate result is breakdown of the organism. This may explain why sleep is universal, from the lowest jellyfish. We need to sleep and if we don’t, we die — and the same goes for your guppy.