BIOLOGICAL CLOCK: ORCADIAN RHYTHM

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Have you noticed that, even without looking at a watch, we usually wake up at about the same time every morning and feel sleepy and go to bed nearly the same time each night? We do not need to know the time to do all these things. Somehow our body knows the time, as if we have a clock inside. This internal clock is called the biological clock.

It is well known that all living things have a biological clock within them. This is because we live on a planet that rotates once every 24 hours, such that a period of light is followed by a period of darkness within every 24 hour cycle. Our sleep patterns, body temperature, hormonal excretion, body metabolism, and other biological functions fluctuate within this cycle as day becomes night and night becomes day. The biological clock governs our body rhythm repeatedly every day, and this rhythm is known as the arcadian rhythm. Orcadian is a Latin word meaning ‘about a day’.

This circadian rhythm is endogenous and is as if governed by an internal clock which is situated inside our brain. The body functions such as sleep patterns, body temperature, etc. appear to follow this internal clock. This body clock is quite robust and possesses a certain amount of durable inertia, and resetting of this clock may take from a few days to a week to complete. This is illustrated by the interesting case of the fiddler crab.

Scientists have noticed that the fiddler crab of North America changes colour according to whether it is night or day. These crabs are flown by air from New York on the east coast to California on the west coast. Because our earth rotates towards the east, New York on the east coast sees the sunrise many hours earlier than California. When the crabs first arrive in California they continue to change colour as if they were still back in New York; they have

not yet adjusted to the new timing of day and night in California. But after about ft week the crabs adjust to the local time in California and change colour according to this new local time. Obviously the crabs do not wear watches. Their biological clock is gradually reset to the new timing of daylight in California.

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