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Most organisms exhibit circadian rhythms that have a periodicity of about, but not exactly 24 hours. Why is this? And to what extent can variation in periodicity in circadian rhythms among the individuals of the same species (if a variation does indeed exist) influence the level of competition they face in acquiring resources?

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    $\begingroup$ Regarding the first question, probably because as long as the organism uses queues from it's environment (e.g. light levels / temperature / tides) it's rhythm doesn't need to be exactly 24 hours. It just has to be good enough, and the organism will use these queues to adjustment its behavior accordingly. Regarding the second question, it should probably be split off into a different question. $\endgroup$ – p.s.w.g Feb 4 '15 at 18:57
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    $\begingroup$ When you ask “why,” are you interested in the molecular mechanism of this variation or the adaptive “reasons” behind it? The title question is more intuitively the former to me, but the second sentence in the body of the question points more toward the latter. If both, I agree with the prior commenter that two separate questions may be in order. @p.s.w.g: indeed, Zeitgebers. $\endgroup$ – Susan Feb 4 '15 at 23:58
  • $\begingroup$ Can you add a couple of references for your claim that "circadian rhythms that have a periodicity of about, but not exactly 24 hours" (will also show the taxonomic scope)? $\endgroup$ – fileunderwater Feb 5 '15 at 10:44
  • $\begingroup$ @Susan and I am interested in the adaptive reasons and the evolutionary significance behind my questions. I found one answer to my first question - that if all individuals of a population had the exact same periodicity, then they would have to face more competition in acquiring resources (I found this on surfing the net). I wasn't entirely convinced, and hence my second question. Sorry for not making the connectivity clear. $\endgroup$ – Amruta Rajarajan Feb 5 '15 at 12:22
  • $\begingroup$ Perhaps because the optimal cycle of life is not 24 hours due to seasonal changes and migrations? It is quite difficult to make biological oscillators with very long periods precise, and it doesn't need to be, because one can entrain an oscillator with external stimuli. And as you mentioned in the comments, variations can be helpful in many ways. $\endgroup$ – Memming Feb 6 '15 at 13:59
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Biological/organic systems are notorious for imprecision, making a perfect 24-hour cycle practically impossible - nevermind the fact that "hours" are a purely artificial concept that humans invented to help make sense of time.

The simplest "close-enough" way to measure days that doesn't require an implanted clock microchip or heavy computing power is to detect extreme changes in light or temperature associated with transition between day and night.

An extra argument against hard-locking to a 24-hour cycle: Adaptability and migration. If every life form is locked to a strict 24-hour cycle with fixed time slots for sunset and sunrise, this limits migratory adaptation since different regions have different sunset/sunrise and day lengths at different times of the year. Timing a wake-up and sunrise and sleep at sunset (or the reverse among nocturnal species) generally works across most of the planet no matter the season (the only exception being the niche case of a solstice at the poles, for example http://www.slate.com/blogs/bad_astronomy/2016/06/22/why_does_the_sun_never_set_at_the_north_pole_in_the_summer.html )

There's also some evidence that days on Earth are getting longer ( see https://www.space.com/40802-earth-days-longer-moon-movement.html )

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