Infradian rhythmicity comprises essential biological cycles that last longer than 24 hours. Their frequency is thus less than that of a day. Thus, the term derives from the Latin words infra (under) and dies (day). These chronobiological rhythms include, for example, the processes of bird migration, the rutting season and the seasonal change of hair and feathers, which last about a year. They are also referred to as circannual rhythms. Among them fall likewise the winter rest, the sexual cycle as well as rhythms, which last approximately one lunar month (circalunar rhythmics).
What is infradian rhythmicity?
Biological rhythms can be divided into two main types. In addition to infradian, these are the circadian rhythms, which span 24 hours and are most important for humans. They include the sleep-wake rhythm and, for example, the rhythm of plant leaf movements. Ultradian cycles, which are shorter than 24 hours, are also important. They can be exemplified by the feeding cycles of field mice. The semilunar rhythm, on the other hand, is oriented to the tides and is important, for example, for fish and their spawning habits. It extends over 14.25 days and reaches its midpoint between two spring tides. The time of 12.5 hours between low and high tide is called circatidal rhythm. It is particularly followed by the people living on the Wadden Sea.
Function and task
Thanks to modern chronobiology, infradian rhythmicity is now studied in great detail. Many important questions for humans are connected with it in the socio-medical field. The manifold effects of shift work are one example. Furthermore, nowadays many psychotropic drugs influence the daily rhythm of humans. Psychiatric disease patterns sometimes have a serious influence on circadian rhythms. In general, today’s life and work are moving further and further away from the course of the so-called biological clock. In addition to increasing shift work, an increasing lack of light is also a cause of this change. Furthermore, frequent travel across time zones strongly influences circadian rhythms. The connection of these developments with psychiatric diseases such as depression cannot be denied. Chronobiology, as a relatively young science, attempts to explore the effects of disturbed natural rhythms and make them largely controllable. Circalunar rhythms, which are oriented to the phases of the moon, have a special significance in this context. As part of the infradian rhythm, it describes the 29.5-day phase cycle of the moon. The amazing reactions of some animals to these natural rhythms can be well observed, for example, in bristle worms. In the Mediterranean, some species of them mate reliably at full moon. The palolo worm also follows the circalunar rhythms. Shortly before the new moon it pushes off its abdomen. This contains the germ cells and moves to the surface of the water, where sperm and eggs are released for fertilization as the sun rises. Perfectly attuned to the infradian rhythm is also the New World spurdog (Grunion). It spawns in the coastal sand during the nights shortly after the spring tide. At the next high tide, the spawning sites are washed free and the larvae of the animal enter the open sea water. This process is repeated about every two weeks, depending on the full moon and new moon, respectively. The female sexual cycle of humans and animals with ovulation and menstruation also represents a typical infradian rhythm. The male rut is aligned with it. In addition, the infradian rhythm in many mammals is at the same time connected with a certain annual rhythm. This, in turn, cannot be determined in humans.
Diseases and ailments
If the infradian rhythm in humans is interrupted or severely shifted, this can lead to infertility in women, for example. Heavy stress caused by constant shift or night work or extended working hours can make pregnancy more difficult, studies have shown. Women are receptive during periods that are essentially comparable to the cycles of the moon and the tides. When a woman’s infradian rhythm is intact, she releases sex hormones in a way that ensures her fertility at an optimal level. Any serious disturbance of this rhythm is detrimental to fertility as well as to health in general.The human infradian rhythm also includes the permanent alternation between tension and relaxation. If this constant alternation is observed and consciously accepted, it means less stress in the daily work routine. The human metabolism is also accustomed to certain rhythms and reacts with defensive behavior when the usual timing is upset. In accordance with his “inner clock”, every human being regularly needs a rest phase of 20 to 30 minutes after an active phase (work, sports, etc.) of 90 to 120 minutes. If this infradian rhythm is observed, performance remains at the best possible level in the long term. The human organism is naturally accustomed to accepting these rhythms. It draws attention to disturbances of these rhythmic processes with certain reactions such as yawning, sleepiness and lack of concentration. If the desire for a break is ignored, the body produces stress hormones beyond the usual level, which have a more or less negative effect on the circulation and well-being. Viewed over longer periods of time, numerous physical and mental illnesses are promoted in this way.
