But this was the nocturnal hum of the midshipman fish; a courtship call, and the source of a biological secret scientists have now solved. Researchers brought the fish into their lab to work out why they sang at night. The US team's findings are published in the journal Current Biology . Chemical clock The researchers found the singing was controlled by a hormone that helps humans to sleep - melatonin. And looking more closely at how melatonin acts on receptors in different parts of the fish's brain could help explain why it is such a powerful "chemical clock" with a role in the timing of

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