Biological Rhythms

Some biological rhythms occur more than once a day and are called ultradian rhythms. The release of hormones from the male pituitary gland of mammals occurs about every one to two hours during the day. Sleep cycles, that is, the cycle from drowsiness to REM (rapid eye movement, dream sleep) to dozing, then light and deep sleep, and finally slow-wave sleep, is a 90-minute sleep cycle that repeats itself during a night's sleep. Constant breathing and the beating of our hearts are also ultradian rhythms, but these activities are also affected by the daily sleep/wake cycle, which is a circadian rhythm. Heart rate and breathing both slow during sleep.

Circadian rhythms are those that occur once a day and relate to the sun. The one that dominates our activities is the sleep/wake cycle people experience every 24 hours. Body temperature, response to medications, alcohol blood level, alertness, and fatigue all have a daily up and down cycle. Circadian rhythms also control such daily activities as eating. Chronobiologists explain the synchronizing of circadian rhythms to sunlight. This external factor helps the body regulate its daily activities. When people travel in an east or west direction through many time zones, they may suffer from jet lag and will need several days to adjust their sleep/wake cycle to the daylight and darkness in their new environment.

The clock in humans is located in the suprachiasmatic nucleus (SCN), a distinct group of cells found within the hypothalamus. The SCN is only one part of the mechanism by which "time" is kept. There are light receptors found in the retina that have a pathway, called the retinohypothalamic tract, leading to the SCN. The pineal gland is a pea-like structure found behind the hypothalamus in humans. The pineal gland receives information indirectly from the SCN. It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland, which secretes the hormone melatonin in response to this message. Nighttime causes melatonin secretion to rise, while daylight inhibits it. Even when light cues are absent, melatonin is still released in a cyclic manner; yet if the SCN is destroyed, circadian rhythms disappear entirely.

Infradian cycles are monthly cycles, the most common of which is the monthly menstruation of females. Illness and death have been correlated to certain times within infradian cycles, with more deaths occurring in the second half of the menstrual cycle. Research with men has shown that weight fluctuations, hormone levels, growth of beards, body temperature, pain threshold, lung capacity, and physical strength all demonstrate monthly cyclical patterns. More men experience symptoms of prostate enlargement during the new moon period, while more deaths and accidents occur around the time of a full moon.

Lunar cycles are somewhat longer than circadian ones. They last about 24 hours and 50 minutes. Some marine invertebrates, such as the fiddler crab, synchronize daily activity to the tides, which are affected by the moon. The change in water levels during the tides influences the activity of many marine invertebrates (animals without a spinal column).

The longest cycle is of course the life cycle, which has the distinct stages of growth, maturation, decline, and death in all forms of life.

The circannual cycle is a yearly occurring one. Some people and animals periodically gain or lose weight at certain times of the year. Transplant patients respond better to certain drug treatments at certain times of the year than at others.

A disorder called SAD (seasonal affective disorder) afflicts many people during the winter months of the year when the days are shorter. The hormone called melatonin is secreted from the pineal gland, which is located just behind the hypothalamus in the brain. Melatonin affects our moods. Its greatest production is during the night and when there is more darkness during a 24 hour period, as it is in winter, there is a tendency for some people to become depressed. An effective treatment is to expose the person to intense bright light.