Every morning when we wake up and open our eyes we see the sun shining through the window. The light/dark cycle appears to be very important in the resetting of our circadian rhythm. When we open our eyes in the morning, the light stimulates the light-sensitive part of our eyes, the retina. The retinae from both eyes convey the light message along the optic nerves to a central point called the optic chiasma, which is in the middle of the brain stem adjacent to the hypothalamus. Half of this light message crosses the optic chiasma and is relayed to the rear part of the cerebral cortex. Scientists now believe that part of the light message is also relayed to a group of nerve cells in the hypothalamus adjacent to the optic chiasma. This area is called the suprachiasmatic nucleus (SCN) in the hypothalamus and is the site of master control of the circadian rhythm. In animals destruction of the SCN abolishes the circadian rhythm.
It is thought that the SCN possesses an endogenous oscillating mechanism which in free running conditions in man is 25 hours. The SCN is the master oscillator, and it is believed that there are other suboscillators which control hormone rhythm, body temperature rhythm, etc. Hence in cases of jet lag or shift work, the phase maps of the different suboscillators are thrown out of phase with each other. By resetting the master oscillator, the SCN, the circadian rhythm, and the suboscillators are put back into place.
Chronobiologists have recently studied the SCN in detail, both in animals and in man. They have found that if a strong light message is received in the SCN at an hour different to normal sunrise, the SCN is reset into a new circadian rhythm after a few days. Chronobiologists call the light signal the Zeitgeber, synchronizer, or time giver, and the resetting process the entrainment.
The Zeitgeber for crabs that are flown from one coast to the other in the USA is the light/dark cycle of the new location. The Zeitgeber for Dr Charles Czeisler to entrain his jet lag patient to a new circadian rhythm is artificial bright light Dr Thomas Wehr of the National Institute of Mental Health in the USA has been using light treatment and sleep deprivation to treat certain kinds of depressive illness. It is believed that, by adjusting the master oscillator, its suboscillator that modulates mood and depression will also be adjusted and lead to recovery from the depressive illness.
At present, in Australia and New Zealand, a great deal of research is being conducted on the chemistry of the biological clock. Melatonin, a chemical secreted from the pineal gland situated at the base of the brain, has been shown to be closely related to the circadian rhythm. During the night, the SCN relays impulses to the pineal gland and melatonin is secreted into the blood. In the day, sunlight has an inhibitory effect on the SCN, and this stops the pineal gland from releasing melatonin. The concentration of melatonin in the blood hence becomes a good marker of the circadian rhythm.
Melatonin comes from the word melanin, which means skin pigment. In some lower animals, skin colour changes according to the amount of sunlight. At night, there is more melatonin and this contracts the melanophores of the skin, making the skin pale in colour, whereas in the day, with plenty of sunlight, the skin becomes darker. This skin colour change is controlled by the light/dark - , of the circadian rhythm through melatonin.
However, in man, the exact role of melatonin is stillunknown. It has been suggested that there is a melatonin stimulates the SCN to secrete even more melatonin and a rise in melatonin concentration speeds up the resetting of the biological clock. A group of volunteers were asked to travel from New Zealand to London and back, and were given melatonin capsules to take for a few nights on arrival at their new destination. It appears that this increase in the concentration of melatonin at night speeded up the resetting of their biological clock to the new local time. They felt more alert in the day and their sleep pattern was reset much sooner than if they had not taken melatonin capsules. A new company, called Circadian Technologies, has recently been set up in Melbourne. It plans to produce melatonin capsules on a commercial scale. Of course, this has yet to be approved by the Food and Drug Administration in the USA. Perhaps, one day, overseas travellers will regularly be taking melatonin capsules to minimise their jet lag. Or there may be coin-operated bright light machines available at all major airports to entrain the travellers’ biological clock to the new local time.