Have you noticed that you can make yourself get up early in the morning without the help of an alarm clock when you have exams? What makes you wake up early? 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 8 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. Do these rhythms depend on a sort of in-built 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 woke up at the usual time. Only few lived on irregular schedules. On the basis of these experiments, we may conclude that our body has an in-built clock. In fact, the clock does not exist literally. The most probable answer could be that our body responds involuntarily to a fixed time for all actions or even to any conceived time. 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 preset time in mind, it works accordingly. Researches are going on this subject and it is hoped that some day scientists would be able to find a definite answer to these questions related with in- built clocks in our bodies. Even though the conscious part of the mind is totally at rest during sleep, the subconscious part of our mind continues to remain alert. This means that the brain may send time signals during deep sleep also.