Biological clocks

Planet earth has its rhythms. It orbits the sun once every 365 days giving rise to the changing seasons, and spins on its axis every 24 hours giving rise to day and night.

Almost all living things have internal clocks that enable them to synchronise their behaviour and physiological processes to nature's rhythms.

Evidence for the existence of biological clocks goes back a long way. In the 1700s, French scientist Jean Jacques Ortous De Mairan was curious to find out what made the leaves of plants rigid during the day but limp at night.

He placed a mimosa plant in a dark cupboard and was surprised to observe that its leaves continued to droop and stiffen with a daily rhythm despite the absence of daylight.

The plant's leaves were not merely reacting to changing light levels, but appeared to be controlled by some kind of inbuilt timer.

The common cockroach is most active during the first few hours of darkness. But studies have shown that cockroaches kept in constant darkness and at a controlled temperature continue to concentrate their activity into two- to three-hour bursts, which recur approximately every 24 hours.

Their behaviour is driven by an internal clock that mimics the day-to-night rhythm of the outside world.

Not only the mimosa and the cockroach, but practically all living creatures show daily cycles in their physiological processes. These are known as circadian rhythms (from the Latin: circa, about; diem, a day).

In mammals, the location of the body's master clock has been narrowed down to a clump of about 20,000 cells in the suprachiasmatic nuclei (SCN), a part of the hypothalamus at the base of the brain.

The cells in the SCN produce a rhythmic output of electrical and hormonal signals that drive the animal's circadian rhythms.

» Next: The circadian clock in humans