For something that takes up a third of our life, we actually know very little about why we need sleep – though we do know that it is essential, and that we struggle without it.
Sleep has a vague definition: having a state of immobility that can quickly be reversed, combined with a greatly reduced rate of responsiveness to stimuli. Due to the difficulties in explaining what sleep actually is and the lack of research outside of birds and mammals, scientists tend to be a little hesitant when attributing the word sleep to reptiles, fish and especially invertebrates, and prefer to use the term rest instead.
However research in flies, zebrafish and and nematode worms show changes in gene expression show differences between “sleep” and “wakefulness” similar to those seen in mammals, which suggests they do sleep, at least how we define it.
But it’s more complex than that. Some animals like whales and dolphins have unihemispheric sleep, which means one half of their brain can rest while the other half remains alert.
So there’s still a lot to learn.
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