Biological Clock.

You must have all experienced that you tend to wake up almost at the same time everyday in the morning and feeling sleepy as the night falls or why do we so often get up before the alarm is due to sound? Because there is a built in clock inside your body, which regulates sleep – wake timings. All organisms possess an innate biological clock (Pittern drigh 1960).
Rhythms of various kinds have been in the biological world for quite sometimes. It was during Great Alexander’s march to India that one of his officers Androsthenes noted a daily leaf movement among the plants of Tamrind. Rhythmic events occur everywhere in the environment; the sun rises and sets forming days and nights. The bird sings mostly in the morning and evenings.
You must have noticed that your pets especially dogs and cats lose more hair during summers, in winters their fur coat gets thicker.
The most outstanding work on biological clocks was done by Erwin Bunning in 1936.Biological clocks are also known as internal clocks or circadian rhythms are probably a biochemical mechanism. These rhythms allow the animals to adopt their behavior in best possible way to the course of day’s changing event.
Where is the master clock located?
Efforts have been made to locate the master clock of animals in the nervous system. In one study, destruction of neurosecretory cells of brain in the region called pars intercerebralis (PIC) abolished the insect’ clock. The many rhythms studied in vertebrates also. Studies of sparrows exhibit the rhythmic activity have revealed that the pineal body. Sparrows that have their pineal body removed and are kept in darkness continue their activity, but no longer n a rhythmic pattern.
But what generates the rhythm is as yet unknown.
Sources: Animal behavior by Mohan Arora. And Reena Mathur.