The role of endogenous and exogenous factors in bodily rhytms
The suprachiasmatic nucleus
In mammals, the main endogenous pacemaker is a tiny cluster of nerve cells called the suprachiasmatic nucleus (SCN), which lies in the hypothalamus. It is located just above the place where the optic nerve from each eye cross over (Called the optic chiasm - thus 'supra', which means 'above' - chiasm). The SCN obtains information about light from the eye via the optic nerve. this happens even when our eyes are shut because light penerates the eyelids. If our endogenous clock is running slow (e.g. the sun rises earlier than it did the day before), morning light automatically shifts the clock ahead, putting the rhythm in tune with the world.
In fact, each SCN is actually a pair of structures, one in each hemisphere of the brain, and each of these is divided inot a ventral and dorsal SCN. The ventral (forward) SCN is relatively quickly reset by external cues whereas the dorsal (rear) SCN is much less affected by light and therefore more resistant to being reset (Albus et al 2005).
The pineal gland and melatonin
The SCN sends signals to the pineal gland, directing it to increse production of the hormone melatonin at night. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness. In birds and reptiles the pineal gland lies just beneath the bone of the skull and is directly regulated by light; light inhibits the production of melatonin. In fact many lizards have a 'third eye' near the pineal gland which actually protrudes through a small opening in the skull and receives information about light.
The process of resetting the biological clock with exogenous zeitgebers is known as entrainment. The opposite of entrainment is 'free-running' - the biological clock operates in the absence of any exogenous cues.
Light is the dominant zeitgerber in humans. As we have seen, light can reset the body'd main pacemaker, the SCN. It also can reset the other oscillators located throughout the body because one of the proteins in the biological clock is CRYY (cryptochrome), which is light sensitive. This may explain why Campbell annd Murphy (1998) found that shining light on the back of participants kness shifted their circadian rhythms.
Until fairly recently, biologists thought that social cues were the main zeitgebers for human circadian rhytms - we eat emals at socially determined meal times, we go to bed and wake up at times designated as appropriate for our age, and so on. Our daily rhythms appeared to be entrained by social convention, not internal biology. Today we know that light, not social cues, is the dominant zeitgeber. However, it is also now understood that all parts of the body produce their own oscillating rhythms and some of these are not primarily reset by light…