I have always wondered how I wake up, on my own, at precisely the same time everyday and how I feel hungry at precise timings, as though I am programmed by some internal alarm clock. That internal clock of the body does indeed exist and is called the suprachiasmatic nucleus.
Our body is a very advanced system which has provision for all our needs. Through evolution, our bodies have developed a timetable for all our internal functions, based on the 24-hour cycle of day and night. The implementation and synchronization of the timetable, according to external stimuli, is controlled by the nucleus, which is a part of our brain.
It is strategically situated in the optic chiasm, which is the visual processing center of the brain and on the borderline that divides the two halves of our brain. It is an elliptical shaped region, smaller in size than a pea. It is closely connected and in interaction with other critical centers of the brain. Actually, it is placed in front part of the hypothalamus too, which is a coordinating link between the neural and the endocrine systems. It has various types of cells in it, catering to different functions. There are oscillator cells in the nucleus that maintain time and photoinduced cells. It interacts with other parts through neurotransmitters and has several peptide linkages.
Just as every clock is synchronized with reference to other clocks, our body clock needs a reference to get its time right. That reference is provided by light sensitivity of the eye. Thus, light is the 'zeitgeber'(German for time-giver) of the body. The light signals received by the eyes are detected by special ganglion cells in the retina. They transmit this signal through the retinohypothalamic track to the suprachiasmatic nucleus. This signal is then relayed to another special cell group called the dorsomedial nucleus in the hypothalamus. This nucleus is essentially the planning commission in the brain, which plans the sleep cycle, the hunger cycle, and the hormone release cycle of the body. Still, the main biological clock is the nucleus, also known as the pacemaker of the circadian cycle.
Once the nucleus adjusts its time, it transmits the synchronization signal to other secondary timekeeping centers, sometimes called 'slave oscillators', situated in the various organs which accordingly adjust their own circadian cycles.
The nucleus carries out many more functions. It sends information to other control centers in the hypothalamus and the pineal gland, that regulates the temperature of the body. It also prompts the release of certain hormones like melatonin (which plays a big role in the regulation of circadian rhythms) as well as cortisol (called the 'stress hormone'). This is how it tunes all the secondary cycles of the body.
All these functions of the nucleus are carried out by special types of neurons which have the ability of photo-induced gene expression. There is a feedback mechanism that can readjust the cycle, according to change of light in surroundings. The photo signals that it receives are relayed through this photoinduced gene expression mechanism, to produce appropriate adjustments. These genes are called 'clock genes'. This sets the body clock, back in phase with the circadian rhythm and this process is called entrainment.
If in some way, the nucleus is damaged, the person's biological clock loses track and his lifestyle is severely affected. He cannot adjust to changing conditions of the day, as his feedback mechanism is compromised, which causes sleep disorders.
In olden days, there was no electricity and, therefore, lifestyles in those societies were totally controlled by the daylight time or the circadian rhythm. Today, with a 24-hour electric supply, illumination is available throughout the day. This upsets our biological rhythms as they are totally photosensitive. Regulating our exposure to light is, therefore, very important. Thus, the nucleus keeps us in tune with the world and sets the pace of our life.