If you have ever flown across several time zones, you have experienced jet lag. You arrived in a new time zone, but your body was still living on the time in the old zone. You were not sleepy at all and were ready for dinner in the middle of the night, and you wanted to sleep all day.
People experience jet lag because all living things have a biological clock. Plants and animals are all in rhythm with the natural divisions of time -- day and night and the seasons.
At sunrise, plants open their leaves and begin producing food. At night, they rest. In the temperate zone of the earth, trees lose their leaves in fall as the days become shorter and there is less sunlight. In the spring, leaves and flowers begin growing again as the days become longer.
Rain sets the rhythm of desert plants. Plants in the desert may look dead for months or even years, but when it begins to rain, the plants seem to come to life suddenly. The leaves turn green, and flowers appear. The plants produce seeds quickly, before the rain stops. These seeds may lie on the ground for years before the rain starts the cycle of life again. The plants' biological clock gave the signal for these things to happen.
At sunrise, most birds wake up and start singing. When the sun goes down, they go to sleep. When spring arrives, they start looking for a mate. When winter comes, some birds fly to a place with a warmer climate. Their biological clocks tell them the time to do all of these things･
Animals live near the sea and get their food from both the land and water have biological clocks set with the tides. When the tide goes out, they know it is time to look for the food that the sea left behind it. Some insects, like honeybees, have a very strong sense of time. Some French scientists did an experiment with honeybees. They put sugar water out every morning at 10:00, and the bees came to drink the water at the right time. Then the scientists started putting the sugar water out at 8:00 p.m. It took the bees a week to find it at the different hour, but from then on, they came to eat in the evening instead of in the morning.
Later the scientists took the honeybees to New York. The bees came for the food at the time their bodies told them, though it was 3:00 p.m. New York time. Their bodies were still on Paris time.
Humans, like other animals, have a biological clock that tells us when to sleep and eat. It causes other changes, too. Blood pressure is lower at night, the heartbeat is slower, and the body temperature is a little lower. We even go through several levels of sleep, cycles of deep and light sleep.
Other things happen in cycles, too. More babies are born between midnight and sunrise than at any other time. More natural deaths happen at night, but more heart attacks happen early in the morning. Most deaths from diseases in hospitals happen between midnight and 6:00 a.m. Some police say there are more violent crimes and traffic accidents when there is a full moon.
The honeybees in the experiment reset their biological clock for different feeding hours. Humans do this, too. People who work at night learn to sleep during the day and eat at night. Students who fly across the world to study in another country get used to the new time zone after a few day. When they go home, they change back again. Our bodies are controlled by a biological clock, but we can learn to reset it at a different time.