The biological clock is an in-built timing system found in all living organisms. It times the various activities of the organisms and keeps them in harmony with the regular changes in their surroundings, such as the day-night cycle. The cycle of sleep, wakefulness and many other body activities repeats over a period of roughly 24 hours. That is why it is also known as the circadian rhythm. It works by keeping track of the rhythmic changes in the environment such as day and night, movement of ocean tides, phases of the moon, and seasons of the year. These rhythms can be shifted and the biological clock can be reset by changing, for example, the temperature at which a plant is grown or the duration of light exposure an animal gets.
There can be cycles of more than 24-hour duration too. Human beings have daily, weekly, monthly and yearly biological rhythms. The level of hormones and other chemicals in blood varies over each of these periods.
One of the effects of the biological clock is jet lag felt while travelling long distances by air.