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This is a question that is a little bit hard to define for me. I'm interested in learning more about times of "attractor/entrainment" processes that may take place in the body/brain of mammals, and particularly humans. I want to get a general idea of how long it takes for a mammal to switch into a new mode of operation, be it behavior or physiological change? Is it days, weeks or more?

For example, a polar circle animals may change the color of their fur in response to photoperiod changes. How quickly does the change get registered and take place? Is it days, weeks or months?

Another example might be a mating season of siberian hamster, where their testes weight changes in response to photoperiod duration. How long does it take for a behavior change like that to fully manifest?

I've read that it takes ~21 days for a human to create a habit or break one. Is there any biological basis for this observation?

I'm not sure how to search for information like that, as it seems to me such things might only be incidentally mentioned in scientific articles. Are there some keywords that may help me improve the search results?

Thank you for any input.

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This is still somewhat ambiguous. I think you need to restrict your question to a specific process you are interested in. Learning and switching to a new state can happen on a much shorter time scale (milliseconds to minutes) in the brain. –  Memming Oct 12 '12 at 19:04
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