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Circadian rhythms Most functions of any living system display an approximately 24-hour rhythm. Because these rhythms last about a day, they are called circadian rhythms. This word is derived from the latin words for about (circa) and the word for day (dies).
A way to study biological rhythms is to investigate a rodent, walking in a wheel. Computers are used to measure when the rodent is walking in the wheel, and on the basis of these measurements you can see when the rodent is active. The results of an experiment with a hamster are shown in the picture. Like most rodents, hamsters are nocturnal, contrary to human beings who are diurnal. A remarkable thing is that although all psychological measures (hormone levels, body temperature) vary across the day, the carcadian show extraordinary precision. For human beings, this would not be so remarkable, because we have watches. However, also animals are attending to a 'biological clock'. If a hamster is blinded or placed in a dark environment, it continues to show a daily rhythm in wheel running, despite the absence of light and dark. This suggests that animals have an internal clock to regulate their activities. However, when blinded the hamster shows a bit of imprecision in its cycle; each day he starts a few minutes later with its activities. That is why scientists now think that his internal cycle is a bit longer then 24 hours. You can see this in the picture, when there is no light and dark difference. He will start each day a bit later with his wheel running. Light Normally our internal clock is reset by light. If we expose a nocturnal animal to periodic light and dark, he will soon adapt his wheel running to the dark period. In humans, circadian rhythms are also entrained by social stimuli. And when you should do the same experiment as done with a hamster with a human, he will reveal the same characteristics. In humans the internal clock cycle is also a bit longer than 24 hours. |
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