When we decide to get up early in the morning at a fixed time, we wake up without the help of an alarm or even a watch. Our body in fact does have a regular daily rhythm and this makes us aware of time. We usually sleep for eight hours every night. Depending upon the temperature of the surroundings our body temperature also undergoes a change and so does the speed of our heartbeat.
Now the question arises, whether these rhythms depend on a sort of built-in-clock or on the daily changes of light and darkness? A famous experiment gave some clues to scientists to find an answer to this question. For several months the researchers lived alone in a cave where they did not even see daylight and had no idea about the time. They camped in tents, with gas heaters, lights, books and even a record player to make life in the cave bearable. Frequently they used a special phone to call the base camp above ground, reporting when they woke up, ate and went to bed.
In another experiment people lived in specially built underground apartments. They, too, reported regularly to scientists outside.
Most of the researchers in these experiments slept and wake up regularly on time. Only few lived on irregular schedules.
On the basis of these experiments, we may conclude that our body has a built-in-clock. In fact the clock does not exist literarily. The most probable answer could be that our body responds involuntarily to a fixed regular time of all actions or even to any conceived time in mind. The actions in response to time may be the results of involuntary body actions or conditioned reflex actions. When the body is accustomed to a particular time for a particular action or become responsive to a pre-set time in mind then it works accordingly in set times. Researchers are going on this subject and it is hoped that someday scientists would be able to find a definite answer to these questions related with built-in-clocks in our bodies.