The Period, or "Per, " gene is one of the most important clock genes known in fruit flies and mammals.

BBC: Body clock 'set before birth'

Six of the clock genes were suppressed by the cytokine in ways that would, in an actual animal, be expected to promote sleep.

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Meanwhile the natural body clock was disturbed - some genes naturally wax and wane in activity through the day, but this effect was dulled by sleep deprivation.

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The genes that run the circadian clock are not yet fully understood.

BBC: Body clock 'set before birth'

Researchers have developed methods of inserting genes to "turn back the clock" on cells that have already specialized, so that they can turn into anything again.

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The research team took fruit flies and removed several genes that regulate their internal biological clock, which, in humans, decides when we wake up and go to sleep.

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"Because mutations start occurring at regular intervals in these genes as soon as a new species evolves - like the ticking of a clock - we can use them to trace the evolutionary history of a species back to its time of origin, " said Dr Hedges.

BBC: Earth AP