Science matters – tick tock

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By Dorothy H Crawford

This year’s Nobel Prize in Physiology or Medicine went to three US scientists whose work led to the unravelling of the biological clock.

They and their colleagues discovered a key gene in the complex clockwork mechanism in the 1970s.

The biological clock is our inner timepiece. It evolved billions of years ago in response to the only constants on our planet – the rotations of the earth and moon. These define night and day, the changing seasons and the tides.

All living things have biological clocks that ensure peak activity when food, and/or sunlight is available and maximise the chances of reproduction.  Circadian clocks control daily rhythms (circa meaning about, diem a day) like sleeping and waking, the biting practices of mosquitoes and the opening of flowers. Longer term clocks, responding to day length or phases of the moon, determine processes like germination, hibernation, migration and reproductive cycles. The extraordinary synchronised reproduction of cicadas, causing young to emerge in strict seven or 13-year cycles, is an extreme example.

Our internal clock is triggered by sunlight. In mammals, photoreceptors in the eye send signals to specialised cells in a region of the brain called the hypothalamus – central control for the biological clock. Inside these cells, levels of unique protein components rise each night and fall during the day in a self-perpetuating cycle that resets itself at sunrise and sunset. This regulates all our internal rhythms including hormone levels, blood pressure, heart rate and emotions.

Interestingly, horseshoe crabs, which evolved more than 350 million years ago, have additional light receptors in their tails – essential since this is their only organ not submerged in mud!

Peak physiological activity during the day means athletes perform better, wounds heal more quickly and fewer major accidents occur during daylight than at night. But only humans attempt to break the natural rhythms, perhaps by working nightshifts or by taking long-haul flights. This disruption may produce temporary jet-lag or shift-lag, but there is growing evidence that more serious problems like bipolar disorder, schizophrenia and even cancer relate to circadian clock dysfunction.


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