Distinction between circadian cycles and sleep?

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Discussion Overview

The discussion centers on the distinction between circadian cycles and sleep in biological organisms, exploring whether these concepts are synonymous or fundamentally different. It examines the implications of these definitions for understanding the evolution of sleep, particularly in organisms with primitive nervous systems.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants question whether circadian cycles, observed in simple life forms like paramecia, can be equated with sleep, suggesting a need for technical differentiation.
  • Others argue that the evolution of sleep is linked to the development of central nervous systems, proposing that circadian cycles predate sleep in evolutionary history.
  • A participant highlights that if circadian cycles and sleep are indistinguishable, then the assertion that sleep requires a primitive central nervous system becomes problematic, given that single-celled organisms exhibit circadian rhythms.
  • It is noted that circadian cycles encompass a variety of activities beyond sleep, including those in bacteria and plants, which do not possess nervous systems.
  • One participant emphasizes that the distinction may stem from human definitions rather than natural phenomena, suggesting a conceptual rather than biological issue.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between circadian cycles and sleep, with no consensus reached on whether they are distinct processes or if they overlap significantly.

Contextual Notes

The discussion involves assumptions about the definitions of sleep and circadian cycles, as well as the evolutionary implications of these definitions. There are unresolved questions regarding the nature of sleep in organisms with primitive nervous systems and the classification of biological rhythms.

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

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