Circadian rhythm (thing)

The importance of the circadian timing system cannot be stressed enough. It is involved in the 24-hour variation of virtually every physiological and psychological variable researchers have studied.

This type of rhythm exists in all living things. Humans have circadian rhythms. So do animals, insects, and even plants. They reflect the adaptation of organisms to the many changes that are associated with the rotation of the earth on its axis. These things include light, air pressure, and temperature. There are over 100 bodily functions and behaviors that follow circadian rhythms. These include such things as blood pressure, heart rate, appetite, secretion of hormones and digestive enzymes, sensory acuity, elimination, and the body's response to medication.

In general, our bodies adapt a strict 24-hour schedule. The reason is the external time cues that are all around us. They train our biological rhythms. In order for scientists to identify endogenous circadian rhythms a subject must be isolated from sunlight, clocks, environmental sounds, and to all other cues of time. It has recently been shown that in primate infants, even at a very premature stage, low intensity lighting can regulate the developing clock.

The body’s overall coordinator is the biological clock. This controls the circadian rhythms. It is located in a teardrop shaped cluster of cells in the hypothalamus. This is the SCN (suprachiasmatic nucleus). There are two, one behind each eye. In the back of the eye there are special receptors that transmit information through neural pathways that allow the body to respond to light and dark changes. This causes the SCN to send out a message to the brain to make the body adapt to the changes. This is not the only clock in the body. They are scattered throughout the body. Some operate independently of the SCN, but for most of the circadian rhythms, the SCN is the master pacemaker. It is now thought by scientists that the circadian timing system develops parentally. The SCN is now thought to be present in primates by mid-gestation.

The SCN regulates the fluctuating levels of certain hormones and neurotransmitters. These neurotransmitters and hormones provide feedback that affects how the SCN functions. The ebb and flow of circadian rhythms is not controlled only biologically, there are external cues as well. The most important of these is sunlight.

Circadian rhythms, under normal circumstances can be synchronized just like a wristwatch can. When a person’s (or other living being’s) routine is changed, their circadian rhythms may be thrown out of phase with one another. This is internal desychronization. This often happens when a person is “jet lagged.” Sleep patterns can adjust fairly quickly but temperature and hormone cycles may take several days to get back to normal.