Zebrafish point to an ancient origin of sleep

As one of those people who REALLY love to sleep -and doesn’t get close to enough of it- I am always interested to read on the topic. What I didn’t expect was to read about sleeping fish and how that relates to its origin ¹. But let’s get down to the details, because if the finding is interesting: Zebrafish DO sleep!! More interesting even -at least for me, but I am a microscopy NERD- is how they came to the conclusion.

To be able to determine if the little fishes were effectively doing something similar to our sleep the researchers, Louis Leung und colleagues built a DIY light-sheet microscope that could image up to single cell resolution. Then, they embedded the fishes in a sort of jelly to immobilise them and be able to record their heart rate, eye movement and muscle tone, as well as record their neutron activity via a fluorescent neuronal marker. So, actually, they built the first fish polysomnograph!! Their recordings show slow-bursting and propagating-wave sleep, which are theoretically similar to slow-wave and REM sleep, respectively. Contrary to mammals, fish do not show eye movements during sleep, but they do show other indicators of sleep like muscle relaxation, a slower heart rate and diminished awareness. Now, the authors consider these facts enough to draw the similarities to mammalian sleep and shift back the date of first sleep introduction in evolutionary history to about 450 million years ago. That’s nothing! How sure can we be that this sleep is a precursor of our sleep? Well, not too sure if we listen to some experts on the field. First off, the experiments were done on infant fish. And, as anyone with kids in the house will know, children’s sleep patterns are quite different from those of adults. And even in the mammalian world, there are huge differences in sleep patterns from one animal to another, from needs of minutes to almost half a day of sleeping. Therefore, a comparison to fish sleeping patterns might be difficult. But that is not to say that fish sleep was not a step in the evolution of sleep. Even if some argue we cannot infer “sleep” from the data, the coolest part of the study is the technical progress that allowed to record this cell behaviour and that could, at least potentially, be used to characterise, in vivo, the effects of drugs over neurons involved in sleeping processes. Be it as it may. I am really impressed by what these people achieved and I take off my hat for them. Now, I think I will go take a nap in honour of our little sleepy friends.