They inserted firefly genes into a fruit fly and found clocks ticking in the wings, the legs, even the proboscis. So far, nine versions of clock genes have been found in humans, discoveries that may help research into jet lag, sleep disorders and shift work. It remains to be proved how they work together, however. But like a watch, the circadian clock must be synchronized to local time. It is critical to a creature's survival that its clock follow the local day-night cycle to be in sync with its prey, its pollinators and other members of its social group.
Bright light acts as a reset button, triggering the molecular activity to start earlier or later.
When an animal is subjected to a new light-dark pattern, its clock adapts. These as-yet unidentified sensors are different from the rods and cones of our vision system. The light message travels along a pathway, called the retinohypothalamic tract, to the suprachiasmatic nuclei, or SCN, in the hypothalamus of the brain. The pineal gland, a pea-like organ behind the hypothalamus, responds to the message by either secreting or inhibiting the secretion of the hormone melatonin. Melatonin levels increase during the night and are at their lowest in bright daylight.