Sleeping with the fishes: the evolution of REM cycles

A new study has revealed that Zebrafish undergo remarkably similar sleep patterns to the REM cycles of humans and other animals. It shows us that sleep is a much bigger part of our evolutionary history than previously thought.

Zebrafish have been shown to have very similar sleep patterns to humans

Sleep is one of the last great mysteries in the natural world. We know what happens to us in our brain and body when we are asleep. But the biological function and evolutionary origin of nodding off is still widely debated by neuroscientists. One way researchers look to get to the bottom of the mystery is by studying sleep in other animals. Some of the more closely related species do share similar patterns, or REM cycles, to us. But a new study has found surprisingly similar patterns of sleep in one of our most distant relatives, zebrafish. Using ground-breaking technology scientists have been able to peer inside the tiny fish to find that they sleep in a very familiar way. The discovery significantly changes what we thought we knew about the evolution of sleep and why it occurred and sets up some interesting research questions for the future.

What is REM?                                                                                      

Restless Eye Movement, commonly referred to as REM, is one of the two main states of sleep in humans. The other state being non-REM sleep. In humans we begin sleeping in the non-REM state where our heartbeat, brain activity and breathing decrease to a point where we are completely relaxed. But when we transition to the REM state our heartbeat, brain activity and breathing strongly increase. In the REM state our bodies are much more active and even our eyes begin to move around underneath our eyelids. This is believed to be the state in which we dream and during REM our leg and arm muscles can be temporarily paralysed to stop us living out those dreams. The origin of REM sleep is a big mystery as it does not serve an obvious purpose whereas non-REM sleep is believed to allow us to conserve energy and undergo necessary ‘housekeeping’ processes by resting.

brain scan
A comparison of brain activity in and out of REM sleep

We are not the only animal that undergoes cycles of REM and non-REM sleep. It can be seen in a wide variety of animals including mammals, birds and lizards. But they can take different forms and last for conflicting periods of time. Just in mammals the length of sleep can span from 3 to 21 hours in a day with almost complete REM sleep or very little at all. REM cycles can also change through an individual’s life, as it has been found in both humans and some other animals that REM sleep becomes less frequent with age. All these things make it very hard to pinpoint why REM sleep evolved in the natural world and how long ago it happened.

Seeing is believing

However we are now possibly a step closer to solving the mysteries of REM sleep thanks to a surprising new study, published in Nature, looking into the sleep of zebrafish. The experiment was headed up by Louis Leung from Stanford University who developed an exciting new way of studying the brain activity in these tiny fish. When signals are sent along nerve cells, or neurons, in zebrafish there is a build-up of calcium caused by a chemical imbalance. So the team genetically engineered zebrafish in the lab to produce a protein that flashes fluorescent green in the presence of calcium. Leung then engineered a microscope capable of detecting the fluorescent flashes of neuron activity in juvenile zebrafish, who have transparent skin before reaching maturity. This meant he and his team could humanely study neuron activity by looking inside the fish without having to implant electrodes.

An example of fluorescence brain activity in the juvenile zebrafish

The experiments involved placing the juvenile zebrafish in a gelatin-like substrate to keep them in place whilst Leung and his team watched them through their specially designed microscope. The fluorescent flashes allowed researchers to watch their neural signals and resulting muscle activity. While being able to look inside the fish meant that heart rate and eye movement that were also visible. The changes in their physiology revealed a surprising cycles of high and low activity associated with REM-like cycles. Leung told National Geographic that the behaviour “just about took my breath away” but that they needed to encourage more sleeping to be able to tell for sure. So the zebrafish were then not allowed to rest for prolonged periods to prevent them from ‘napping’ so they would become more tired. Once they then fell into a heavy sleep they were tested again. The results were the same neural patterns but much stronger and more defined allowing the team to confirm the sleep cycles were genuine in the zebrafish.

The evolution of nodding off

Although the experiment and results of this study are very interesting, you may be wondering how this changes our understanding of sleep. Well, because of how distantly related zebrafish are from other cyclic sleepers like humans, it means that sleep as we know it evolved much earlier in our shared evolutionary history than we previously thought. To be more precise researchers now suggest that sleep must have evolved at least 450 million years ago, as this was the time we diverged from fish. It also throws some doubt into some popular theories of why sleep, in particular REM sleep, originated in nature. One of the most popular theories behind REM sleep is that it helps warm blooded animals, like us, to control their body temperature. After all we sleep for longer, particularly in the REM stage, in colder conditions. But if zebrafish have similar patterns of REM-like sleep this casts major doubt that it is involved in thermoregulation because fish are cold blooded.

Zebrafish have changed the way we think about the evolution of REM sleep

Another popular theory is that REM sleep is a result of high intelligence and brain activity. It has long been thought it also plays a part in memory retention and that it is why we and other intelligent animals are able to do it. But this finding also makes this seem unlikely as zebrafish are comparatively very unintelligent with little capacity for forming memories. REM sleep is also completely absent from more intelligent marine life like cephalopods whose brains are extremely complex compared to fish. The findings could support the idea that REM sleep is important in development as the individuals were all juvenile. But because the same study cannot be done on adults we cannot know for sure. Regardless the study and its findings are very important, not because they answer any of the biggest questions about REM sleep, but because they disprove lots of our original ideas.

What’s next?

The work done by Leung and his team has received large amounts of praise from the neurological community since their work was published. Not only has it opened up more avenues of research regarding REM sleep it has also introduced a fantastic study method which could be used to conduct lots more neurological studies on zebrafish in the future. However not everyone is convinced and some scientists are still sceptical that the findings are as conclusive as Leung and his team make out. Therefore more work is needed to see if more REM-like cycles can be found in different fish and other types of marine life. Hopefully future work into how the creatures in our ocean catch their Z’s could help us finally learn more about how we do the same.

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