Are you a 'lark' or an 'owl' - a morning person or an evening person? Do you do things more efficiently in the morning or later in the day? What time will you go to bed tonight? Scientists say it depends on the inner clock which controls your daily rhythm. The behaviour of almost all plants and animals shows regular patterns or rhythms. There seems to be a connection between these rhythms and the revolution of the earth, its orbit of the sun and the moon's orbit of the earth. The daily rhythm of sleeping and waking is obvious, and so is the annual rhythm of plant growth and animal behaviour. Leaves fall in autumn, squirrels sleep most of the winter, birds fly south, and so on. Animals and plants at the edge of the sea show behaviour patterns which are controlled by the rhythm of the tides which are themselves caused by the movement of the moon. The daily rhythm in humans varies little over long periods, and the body changes irregularly in the same way every day. Body temperature is not a constant 37°C. It is lowest at about 4 a.m. and then rises to its highest point in the afternoon. As it rises, the heart beats faster, and other changes take place. Body temperature rises more quickly in 'larks', but it falls again sooner than in 'owls', whose best time is later in the day. Experiments have been done with volunteers who have agreed to spend weeks alone in an environment of constant light and air temperature, without any contact with the outside world, and without clocks or watches. In these conditions the length of their day gradually increases until they reach their natural cycle, which is usually between 24 and 25 hours. When the experiment is over, the volunteer usually discovers that rather more time has actually passed than he or she had imagined. The same effect has also been noticed in prisoners who have been kept constantly in the dark. The existence of the daily cycle is one reason why people who fly long distances east-west can take several days to get used to the time change. At first the inner clock still behaves as if it was the middle of the night, even though it may actually be lunch time. This can be unfortunate, as there is evidence that the daily cycle influences resistance to illness as well as general efficiency and mood. 1. Answer the questions in complete sentences. Do not copy the text word for word. 1. What causes the movement of the sea? 2. Is your body temperature higher in the day or at night? 3. An owl is a kind of bird that is active at night. What is the meaning of the word 'owl' as it is used in the text? 4. What was the aim of the experiment when someone was shut away from the outside world for a period of time? 5. Why does flying long distances east-west give us more problems than flying north-south? 2. Complete these definitions and explanations. Use a word from the text. 1. An ___ example means an example that can be easily and clearly understood. 2. A noise that never stops is a ___ noise. 3. A person who willingly offers to do an unpleasant job is called a ___. 4. The things you can see all around you are your ___. 5. A ___ is someone whose freedom has been taken away. 3. Complete this short text. Use the words in brackets in the correct form. Changes in our body (control) by an inner clock. Almost all plants and animals change their behaviour (regular) on a (day) or annual basis. Some humans reach their maximum efficiency (early) in the day than others do. Over the last few years scientists (succeed) in (find out) more about our inner clock. Its existence (be) one reason why business people should try (avoid, take) important decisions soon after a long flight.

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