6.13: Biorhythms

Many activities in the body show regular cyclic fluctuations or biorhythms. One of these is a daily biorhythm that repeats itself approximately every 24 hours. It is aptly called the circadian rhythm, meaning "approximately daily rhythm. Perhaps the most obvious manifestation is the cycle of sleeping and being awake. Another well-known biorhythm is the menstrual cycle in women, which recurs approximately every 28 days. Faster cycles include the cardiac cycle and the breathing cycle. People also exhibit annual rhythms that accompany seasons of the year.

In the body, the circadian rhythm is controlled primarily by the brain. When light entering the eyes causes impulses to be sent to the brain, many of the impulses reach a brain area called the suprachiasmatic nucleus (SCN). The SCN is in the hypothalamus, located between the basal ganglia (Figure 6.8, Figure 14.1). Impulses from the SCN travel an indirect route to the pineal gland of the brain. The pineal gland is in the crevice between the cerebral hemisphere and the cerebellum (Figure 6.8, Figure 14.1). The pineal gland secretes the hormone melatonin. When less light enters the eyes, more impulses travel from the SCN to the pineal, causing more melatonin secretion. More light entering the eyes causes the opposite effect. Both the intensity and wavelengths of light influence its effects on melatonin secretion.

Since usually more light enters the eyes during the day and light decreases during the evening, remaining very low during the night, melatonin secretion increases during the evening and remains low during the day. Melatonin influences many body functions including the SCN, and it produces some manifestations of the circadian rhythm. However, even with no light entering the eyes, the SCN causes melatonin to be secreted in a circadian rhythm. The SCN in the main regulator of the body's circadian rhythm. The circadian rhythm is influenced by other factors including environmental cues, physical activity, and eating.

Body circadian rhythms include sleep:wakefulness; stages of sleep; lowering of body temperature, blood pressure, and urine production at night; and oscillations in blood levels of many substances including hormones (e.g., melatonin, glucocorticoids, growth hormone, testosterone, estrogen, progesterone). Oscillations of these hormones cause manifestations of the circadian rhythm (see Chapter 14). The importance of maintaining normal circadian rhythms is evident when they are disrupted. Examples include "jet lag,” working night shifts, or having sleep:wakefulness cycles disrupted by environmental irregularities (e.g., nighttime noise).

Aging causes changes in circadian rhythms. Many changes begin during the third decade and increase after that through old age. In general, manifestations of the circadian rhythm have lower peak intensities. Examples include difficulty falling asleep; poorer sleep quality; more urine production at night; and lower peak hormone levels.

The circadian rhythm tends to shorten, and most manifestations begin up to one hour earlier in the 24-hour day. However, phase shifts are unequal, and some manifestations of the circadian rhythm occur later rather than earlier. The result is an age-related loss of synchrony among manifestations of the circadian rhythm. Perhaps the most obvious troublesome consequence is the age-related deterioration of the sleep:wakefulness cycle accompanied by deterioration of sleep quality.

Age changes in circadian rhythms may be due to a combination of age changes in the brain and the eyes. Weak or disrupted circadian rhythms can be brought toward normal by regulating exposure to bright light, by voluntarily regulating routines (e.g., physical activity), and by carefully timed melatonin supplementation.

In general, there are only small age changes in seasonal rhythms. Exceptions include levels of clinically important substances in the blood (e.g., creatinine, urea, urate, blood proteins).

Understanding and accounting for age-related changes in circadian rhythms and seasonal rhythms are important because circadian rhythms influence patient evaluations and effects of medications. The changes should also be considered in research studies so that measurements are taken at proper times of the 24-hour day.