I was reading this fascinating article about how sleep evolved before brains, and it got me thinking "can plants sleep" and "what is sleep anyway".

A line in the article states:

Studies by a team in South Korea and Japan showed that the hydra periodically drops into a rest state that meets the essential criteria for sleep.

But after looking around I am unable to find the "essential criteria for sleep" - Before I am able to answer questions like "can plants sleep" I think I first need to understand what these essential criteria for sleep actually are.

Can anyone here help me out? Many thanks in advance.

  • $\begingroup$ I have edited your question because criteria is a Greek plural (the singular is criterion). This is relevant to your question because the answer to your question is not a single thing. (btw Have you researched this at all? I imagine there is a wide literature on this topic.) $\endgroup$
    – David
    May 19, 2021 at 21:35

1 Answer 1


I take it you did not even read the article. The article quite nicely expounds on this question. The whole idea is that sleep used to be considered as a human or psychological phenomenon that could only be studied by EEG, and clearly sleep was a phenomenon that goes deeper and wider in biology than just EEG waves. From the same article, just a little below:

She distilled a set of behavioral criteria to identify sleep without the EEG. A sleeping animal does not move around. It is harder to rouse than one that’s simply resting. It may take on a different pose than when awake, or it may seek out a specific location for sleep. Once awakened it behaves normally rather than sluggishly. And Tobler added a criterion of her own, drawn from her work with rats: A sleeping animal that has been disturbed will later sleep longer or more deeply than usual, a phenomenon called sleep homeostasis.

The keyword is behavioral criteria.

This is cited, I recommend a read-through.

I also recommend reading the rest of the article, it guides you through the status quo in sleep science which may further answer more of your questions. At the end, there is an exposition on animals without brains that seem to be sleeping. The hydra is one of them.


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