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Comment by yourkin | Hacker News Reader

Not an expert on jellyfish physiology, but might there be a distinction between "a sleep-like state" and being plain "tired"? After doing some physical activity animals tend to need some rest. Exhaustion is much less of an exciting phenomenon than sleep is.

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Sleep is typically defined by the brain waves activities, delta waves, theta waves, etc., each corresponding to a specific sleep stage. These are physiological markers of sleep. There are also states of "quiet wakefulness" that share behavioral characteristics of sleep, like immobility, for example, but are not sleep per se.

Many "simple animals" like a fly, or a caterpillar or a bee have periods of inactivity that could be an example of rest. But sleep and rest are not the same. If we put brain waves aside, we are left with the behavioral definition of sleep: it is a state of reduced awareness of the surroundings, there will be some latency in arousal, it is reversable, and it is associated either with immobility or at least reduced activity. This study obviously used this definition, so it is not exactly being "tired".

Thank you for articulating the difference between the two. Can it not be the case with exhaustion also, that awareness and/or reaction times are reduced? Cognition certainly is affected after strain in higher order animals and muscle cells need to replenish ATP.

Removing brain waves from the equation, it seems like the whole question of what attests for sleep gets blurry.

Sleep is not just "reduced" cognition, it is an entirely different state all together. In sleep thalamus becomes a gatekeeper that makes sure that stimuli cannot get through (although the important ones still do, like a baby cry will nearly always awaken a mother, even a faint cry will do). So, again, it is more radical than "quiet wakefulness" due to exhaustion.

Now, with "lower" animals it gets tricky, because all we have is a behavioral definition of sleep. But still, it makes a distinction of reduced awareness, arousal latency. Can we call it "sleep" in these animals, jellies included? Maybe not, maybe it is some sort of proto-sleep state?

I'm not convinced because your answer doesn't address the nervous activity of these lower animals. Maybe they don't exhibit any at all in that state, but there's no reason to preclude this diffuse, sparse network from exhibiting patterns exhibiting precursors to sleep comparable to at least one of the human sleep phases.

Yes, it's getting interesting. There are probably significant differences in chemical activity in cells that could be quantified and strictly detect in which state the organism is in.

If it were simply physical exhaustion, you'd expect to see some sleeping during the day, or be able to induce exhaustion.

Induced exhaustion is one way to test it, yes. Can't say from the article if they ruled it out. Correlation of the rest pattern with a day-night cycle does not necessarily imply that the "sleep-like" state is indeed sleep, and not just a power-down low-metabolical wait-state used for the bio-chemistry to catch up after a day of activity is over.

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