15 Examples of Biological Rhythms

The biological rhythms they are periodic changes that occur in organisms, at regular intervals of time. For instance: heartbeat, brain waves, menstrual cycle.

All organisms experience biological rhythms, which can be:

  • Extrinsic Rhythms. When the changes are determined by factors external to the body. Triggers can be light, humidity, temperature, alternation between day and night, moon phases, change of season, etc.
  • Intrinsic Rhythms. When the changes are caused by internal phenomena to the organism.

Although some rhythms are considered to be intrinsic because they have been observed in laboratory conditions (isolation of external factors), in the normal development most of the biological rhythms of organisms are affected by both internal and external factors.

They are called synchronizers to environmental factors that are capable of varying endogenous rhythms.

Types of biological rhythms

  • Cardiac rhtyms. They are those that repeat approximately every 24 hours (between 20 and 28 hours). They are associated with the Earth’s rotation and the consequent variation of light. It has been observed in laboratory conditions that they are endogenous, however the intervals are modified by external factors. The endogenous nature of circadian rhythms is due to genetic adaptations that developed in each species. Its origin can be found in the need to protect the DNA replication of the first cells from the ultraviolet radiation they suffer during the day. This would be the first circadian rhythm: nocturnal cell reproduction. Currently, organisms have internal “clocks” that regulate their intrinsic rhythms. In mammals, this clock is found in the suprachiasmatic nucleus, located in the brain (in the hypothalamus, above the optic chiasm). However, circadian rhythms can be severely disorganized by environmental conditions. This is because the activity of the suprachiasmatic nucleus is modulated by external factors, such as lumen variation.
  • Lunar rhythms (also called selenians or multinictemerals). They are related to the movements of the moon. However, they vary depending on whether the change occurs in a certain phase of the Moon, or every lunar cycle or every half lunar cycle.
  • Tidal rhythms. Those who are affected by high or low tide. They affect organisms that live in or near the sea. Indirectly, the tidal rhythms are affected by the gravity that the Moon exerts on the terrestrial water mirrors, for this reason the tidal and lunar rhythms are closely related. Many of the vertebrate sex cycles (which are not seasonal) have a tidal rhythm, due to the periodic secretion of sex hormones.
  • Annual rhythms. Those biological activities that are always repeated at the same time of year. They are considered to be governed by internal (genetic) and external factors (changes in temperature, food availability, etc.). In animals, reproduction usually follows annual rhythms, as well as seasonal migrations. Also other biological rhythms such as hibernation or lethargy are adaptations to periods of extreme temperatures, and therefore are annual.
  • Ultradian rhythms. They have a much shorter periodicity: between 30 minutes and 6 hours. They are associated with motor and eating behaviors, as well as rest / activity cycles. They also govern phases of sleep at certain ages. For example, in newborns the rhythms of dreams are predominantly ultradian. They intervene deeply in learning processes, since they affect the levels of attention. They are affected by other rhythms. They affect the release of some hormones, heart rate, respiratory movement, thermoregulation and appetite (also associated with the release of hormones).

Examples of biological rhythms

  1. Heartbeat. Two-phase pumping action, performed by the heart.
    • Diastole. Once blood collects in the upper chambers of the heart (atria), they contract, causing blood to flow into the lower chambers (ventricles).
    • Systole. When the ventricles are full of blood, they contract and the blood comes out. The right ventricle sends blood to the lungs to oxygenate it, while the left ventricle sends blood to the body to distribute oxygen.

The heartbeat can vary in its rhythm depending on the body’s operating conditions (physical activity, rest) and context (stressful situations, temperature changes). That is to say that it is a biological rhythm only relatively constant.

  1. Respiratory movement. Respiration is associated with the heart rate, since it is a biological rhythm that allows the blood to be oxygenated. There are two types of respiratory movements.
    • Inhalation. Air is drawn into the body. The muscles of the diaphragm contract, curving downward. This creates a vacuum that causes the lungs to swell, allowing air to enter.
    • Exhalation. Air leaves the body. The muscles of the diaphragm relax, causing the lungs to decrease in volume and therefore the air they contained leaves the body.

While the air is in the lungs, gas exchange occurs that allows the blood to oxygenate and eliminate gases that are harmful to the body.

In the same way that occurs with the heart rhythm, the respiratory movement is modified by the body needs, so its rhythm is usually constant but not invariable.

  1. Brain waves. Electrical activities produced by the brain. Their rhythm is measured in cycles per second (Hz). According to each mental state, different types of wave are produced:
    • Spectrum (more than 40 Hz). They were discovered recently, thanks to the development of digital electroencephalography. The function of this faster brain activity is not yet known.
    • Beta (14 to 40 Hz). They occur in normal waking consciousness. Allows alertness, critical reasoning, and logical thinking.
    • Alpha (7.5 to 14 Hz). They occur in states of relaxation, with the eyes closed. These types of waves favor imagination, memory, learning and concentration.
    • Theta (4 to 7.5 Hz). They occur during deep meditation or during light sleep (REM). The subconscious expresses itself through these waves, it is the frequency in which dreams occur.
    • Delta (0.5 to 4 Hz). It is the slowest frequency. It occurs during deep sleep, when there are no dreams. It is necessary for any healing process.
  1. Sleep – wakefulness. Related to the nitameral rhythm (day-night). It depends on the external influences of light, noise and movement that we usually experience during the day. It has been observed that without external influences this rhythm exceeds the duration of one day (from 25 to 29 hours). For this reason, there is the phenomenon of “jet lag”, the change in the rhythm of sleep when traveling to a territory with an alternation of light and darkness very different from the original one. In other words, the synchronizers of this biological rhythm are the alternation of light and darkness and environmental factors (work obligations, activities, etc.).
  2. Menstrual cycle. Process that prepares the uterus of women and female animals for pregnancy. In women, the menstrual cycle lasts 28 days on average (some women have shorter cycles and others longer).
  3. Seasonal affective disorder. It is a mood disorder that appears at a certain time of the year. The most common is that it occurs in winter or at the end of autumn. It is associated with major depressive disorder. There are hypotheses that it is due to a brain response to a decrease in natural light, lowering the levels of serotonin and melatonin (substances that regulate mood).
  4. Crustacean activity on marine beaches. Much of the crustaceans have behaviors that respond to the tidal cycle. For example fiddler crabs gather on mud banks at low tide, digging a hole where they will remain when the tide rises.
  5. Feeding. The sleep-wake rhythm affects all other bodily functions, as it modifies body temperature, blood pressure, and the secretion of hormones such as melatonin. That is why all the organs of the digestive system are also affected. The intestine, for example, is more active during the day. The hormones responsible for regulating intake (leptin and adiponectin) vary depending on the time of day. However, as we have already observed, biological rhythms are affected by factors external to the organism, related to social, work and cultural activities. For this reason, the daily habits of each person activate the digestion mechanisms at the times when they eat regularly.
  6. Reproductive rhythms. Reproductive rhythms vary in each species. For example, most temperate animals have reproductive periods only at certain times of the year. These animals have a seasonal reproduction. This is due to a natural adaptation to the time when the environment is more conducive to the birth of the young.
  7. Seasonal migrations. Seasonal migrations are periodic movements from one habitat to another. Different kinds of animals make seasonal migrations: birds, fish, lobsters, amphibians and mammals. Migrations can have the objective of moving away from extreme climates (that is why they are always carried out at the same time of year) or reaching a place conducive to reproduction (as is usually the case with fish). Migratory movements tend to cover greater distances in birds, which even change from one contain to another (such as swallows that migrate from Europe to Africa).
  8. Hibernation. It is a state of lethargy that allows animals to adapt to extreme cold. It can last for days, weeks or months. It allows them to conserve energy during times when food is scarce, slowing metabolism significantly. Other biological rhythms also decrease during hibernation, such as respiration, heart rate, and brain waves. Among the mammals that hibernate are the marmot, the dormouse, the hedgehog, the ground squirrel, the hamster and the bat.
  9. Winter torpor of reptiles and amphibians. Reptiles are cold-blooded (heterothermic) animals so they usually do not go through a period of hibernation. However, some reptiles and amphibians go through a process similar to hibernation, during which they remain protected in burrows in a state of torpor.
  10. Summer lethargy of desert mammals. While the best known periods of lethargy are hibernation, which occurs in winter, other mammals can defend themselves from extremely high temperatures in the desert through a period of lethargy that occurs during the summer (summertime). The gerbil, for example, goes into a lethargy in times of higher temperatures.
  11. Flowering in plants. In most flowering plants, they begin to grow in early spring. This is due to a natural adaptation, which makes plants genetically ready to flower when temperatures begin to rise. It is not yet known how plants perceive these changes in temperature.
  12. Tubing in plants. Tuberization is the process by which the roots or lower parts of the stem of a plant develop into tubers, such as potato (potato) or sweet potato (sweet potato). The tuberization depends on certain hormones of the plant. The beginning of its growth occurs between 15 and 28 days after sowing, and lasts between 10 and 14 days, usually the days before the flowering of the plant. Although this is a relatively stable biological rhythm, it is affected by both internal factors (whether the plant arises from a new or old seed, for example) and external factors (light, available nutrients, humidity, temperature).