Circadian Rhythms

Circadian rhythms pertain to changes in body function that occur throughout a 24-hour period. Circa is the Latin word for “about,” and diem is the Latin word for “day.” The body operates with many circadian rhythms, such as body temperature regulation, endocrine (gland and hormone) function, airway function, and kidney (renal) function.

For instance, body temperature slowly rises throughout the day, drops dramatically around midnight, and begins to rise again before 6 a.m. These changes in temperature are often felt during fever, when fluctuations are intensified by high body temperature. The body also regulates breathing patterns. So patients with asthma often have more bronchoconstriction and more difficulty with their asthma during the night when airway function fluctuates to compensate for sleep.

Mental alertness and the propensity to fall asleep are regulated by circadian rhythm. There are two peak times of day at which a person is most susceptible to falling asleep, 3 a.m. to 5 a.m. and 3 p.m. to 5 p.m. This circadian tendency has serious ramifications in our society. There are a disproportionate number of automobile accidents between 3 a.m. and 5 a.m. compared to the number of people on the road at these times. This increase in automobile accidents is partly attributable to conflicts between circadian rhythms and, for instance, atypical lifestyle or shift work constraints that put drivers on the road at these times. Decreased vigilance can combine with sleepiness and result in an accident. Apparently, circadian rhythms are evident in all types of activity.

Most people have experienced the urge to fall asleep in the late afternoon, after eating lunch. It is not necessarily the food that makes people want to fall asleep at this time, but the time of day. Again, the routines of environment conflict with circadian rhythms, which can influence one’s ability to function.

All the rhythms mentioned above occur in humans within a cycle of approximately 24 hours. The exact length of the cycle is not known and is probably slightly longer than 24 hours, perhaps 24.7 to 25 hours. These natural rhythms occur even in the absence of daylight or darkness. Studies in which people live without any way of knowing what time of day it is have shown that these people continue to follow a 24-hour schedule. That is, they sleep for about eight hours, and their waking activity levels correlate with those found for individuals under normal conditions of time and light-dark cycles. It seems the influence of circadian rhythm is unavoidable.

People naturally synchronize their internal clocks with day-night cycles, which allows them to be awake during the day and to sleep during the night. The body regulates this chiefly through the eyes. Light stimulates nerves in the retina that pass a signal through a chain of nerves to the brain. Some of these nerves feed the hypothalamus, a part of the brain that regulates body temperature, water and sugar ratios, and fluid secretions and which houses the suprachiasmatic nucleus, a bundle of nerves that controls the body’s circadian rhythms. When, for example, bright light stimulates the optic nerves, the optic nerves send the signal to the suprachiasmatic nucleus, which then triggers circadian rhythms, resulting in the synchronization of the body's 24-hour cycle with the earth's 24 hour cycle.