Distinction between circadian cycles and sleep?

[Aidyan]

TL;DR Summary

Semantic and technical distinction between circadian cycle and sleep.

Is, in biology, the notion of the circadian cycle (or rhythm) that one finds also in elementary forms of life (say a paramecium) the same notion of "sleep"? If not what is its technically difference? If it is, why are then biologist surprised to observed sleep-like states in organism with primitive nervous organization like in the freshwater polyps? https://pubmed.ncbi.nlm.nih.gov/33028524/

 

[jim mcnamara]

The paper is trying to answer: how did sleep evolve for animals with the first central nervous systems?
It is not making claims about other things, other than it makes assumptions - primarily that sleep requires a primitive central nervous system and if you find an "early" enough CNS you can learn about how sleep evolved.

Humans intuitively use "cubby hole systems" to classify things, Biologists are no different. So what you correctly observe is that Circadian cycles in living things are supposed to have existed before sleep.
What is halfway between the two? An as yet undefined (not seen) progenitor for sleep? What if they are all extinct?

Since these are definitions we apply, and living things do not care about our definitions, that breaks the cubby hole method - you need a half-way hole.

 

[Aidyan]

But the statement that "Circadian cycles in living things are supposed to have existed before sleep" automatically implies that circadian cycles and sleep are two very different processes that one should distinguish. What distinguishes it? If there is no distinction, i.e. circadian cycle=sleep, then the assumption that "sleep requires a primitive central nervous system" makes no sense: it is a well known fact that already single cells undergo a circadian cycles.

 

[BillTre]

Bacteria can have circadian activity cycles of various kinds.
Circadian cycles are not limited to just sleep cycles.
Daily sleep cycles are a subset of circadian activity cycles.
Some other activity cycles involve nervous system activity, but not all.

Plants also have circadian cycles. No nervous system there.

 

[jim mcnamara]

It is a human perception/definition problem - not a problem in Nature -- was my point.