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Of course, over the span of a couple days, our circadian rhythms would lengthen a few hours. But what would happen to the circadian rhythms of humans overall if we had no daylight ever? Say we can live for 10,000 years, how would our circadian rhythm be different 10,000 years from now after zero exposure to sunlight? Would it still be slightly more than 24 hours?

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    $\begingroup$ Your question made me curious so I found this study of fossorial species like naked mole rats. With little to no light input from the environment their societies seem to maintain a circadian rhythm: ncbi.nlm.nih.gov/m/pubmed/11134679 $\endgroup$ – Sudachi Jan 5 '18 at 22:54
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Say we can live for 10,000 years, how would our circadian rhythm be different 10,000 years from now after zero exposure to sunlight?

Probably almost the same.

A circadian rhythm has - by definition - a free running period of more or less 24h. Of course this might change over time, but 10,000 years are not very much on an evolutionary time scale.

As an example there are some studies about the circadian clocks of fish that have lived in underground caves for over 1 million years: They still have a circadian rhythm (even though its doesn't react to light anymore), but even after 1million years the free running rhythm only changed from 24h to 47h. Interestingly it can also still be 'entrained' (synchronised) to other regular cues like feeding time (this is also the case for normal circadian rhythms).

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Blindness has the same effect as a total and permanent absence of light: it causes an abnormal melatonin onset, and, consequently, circadian desynchrony and increased wake times. To preserve the circadian rhythm, administer melatonin (dosing and timing matters).

In sighted individuals, artificial light at night disrupts daily rhythms and suppresses nocturnal melatonin production by the pineal gland. This observation could be exploited in your hypothetical scenario: to preserve the circadian rhythm, substitute the missing sunlight by artificial light (dosing and timing matters).

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