Although it sounds like a small-scale question, solving the mystery of ‘do fish sleep?’ demands a deep and surprising dive into aquatic biology. Yes: at surface-level, many fish seem like they are snoozing at times. They become less responsive, with their heart rates slowing and overall movement limited. But this rest isn’t the same as human shut-eye. For starters, fish can’t actually shut their eyes – they don’t have any eyelids. Secondly, there’s one big neurological difference between fish and people. “Regions of the brain become much less active when humans sleep – particularly in the neocortex, the region that processes most high cognition. But fish don’t have a neocortex, so the two brains are very difficult to compare,” explains Dr Michael Webster , marine biologist at the University of St Andrews. “It’s also very hard to know what is happening in a fish’s brain during sleep – you can’t just toss your goldfish into a CT scanner. Most of what scientists know about this topic has been found studying one type of fish, the zebrafish. Their translucent skin means that with a powerful enough microscope you can actually zoom in on individual brain cells and see how they’re […]

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