What is the menstrual cycle?


Before talking about the phases of the menstrual cycle, I must say what it is. The menstrual cycle is the monthly physiological changes in the ovaries and genitals of a woman that occur in order to prepare the body for a possible pregnancy. In this process, not only reproductive organs take part, the brain and hormonal system play an important role.

The entire cycle can be divided into four segments. Some scientists speak about the incorrectness of such a division and single out only two phases of the menstrual cycle: follicular and luteal. In fact, both classifications are almost identical. Most accurately reflects the processes of preparation for a probable pregnancy, the first classification, which distinguishes four phases of the menstrual cycle. About them will be discussed below.

The first two phases reflect preparation for the formation of an egg cell, the last two - the formation of an egg cell, preparation for conception and conception / dying of an egg cell. Consider each of them separately.

The first phase of the cycle begins on the very first day of menstruation. At this time, the endometrium is rejected, and the body directs its attention to the formation of a new egg. By the time this process takes from three to six days, but even before the end of the month comes the next phase, follicular.

The second period of the cycle lasts about two weeks after the monthly period has ended. The cycle on this segment focuses on the development of follicles under the action of a hormone, in one of which (dominant) an egg cell will later mature. At the same time, estrogen renews the lining of the uterus and makes the cervical mucus immune to sperm.

The duration of the next period, the phase of ovulation, is approximately three days. At this time, the level of follicle-stimulating hormones decreases, which give way to the luteinizing, which makes the cervical mucus susceptible to sperm. In addition, this hormone affects the completion of the maturation of the egg, after which it is released from the follicle (ovulation occurs). The mature egg moves to the fallopian tubes, where it awaits the moment of fertilization for about two days.

It is replaced by the fourth phase of the menstrual cycle - luteal. After the egg is released from the follicle, it is sent for fertilization, and the follicle itself begins to produce progesterone. The role of progesterone is to prepare the endometrium of the uterus to absorb a fertilized egg. At the same time, the production of luteinizing hormone is stopped, which again makes the cervical bond immune to sperm. The duration of the fourth phase of the cycle can last up to 16 days. The body is waiting for the uterus to absorb a fertilized egg, which occurs 6-12 days after the moment of fertilization.

If the uterus has taken an egg cell, a special hormone is immediately produced, under the influence of which the corpus luteum will function throughout pregnancy. It is to this hormone that pregnancy tests are sensitive.

If fertilization has not occurred, the egg together with the corpus luteum dies off, progesterone stops being produced, this is inevitably followed by destruction and rejection of the endometrium. In other words, menstruation begins, and all phases of the menstrual cycle are again replaced by one another.

The following fact is remarkable. All four phases of the menstrual cycle develop under the action of hormones that affect not only the physiology, but also the emotional state of a woman. The ancient Chinese believed that the knowledge of the characteristics of each stage of the cycle is directly related to the well-being of women. Thus, the accumulation of energy occurs before ovulation, therefore, if a woman saved energy in the first half of the cycle, it allowed her to live in harmony with herself and the world around her.

The duration of the menstrual cycle

Despite the fact that there are many physiological abnormalities, obstetrician-gynecologists have concluded that the duration of the menstrual cycle should be 28 +/- 7 days. That is, no less than 21 and no more than 35 days. Also the cycle should be regular. If one cycle was 23 days and the other 28, then this is not a variant of the norm. In such cases, you should consult a doctor to determine the cause.

The duration of the menstrual cycle can vary by one to two days, but no more. This is permissible, since many factors affect ovulation (stress, viral disease, long-distance travel, acclimatization). After ovulation, 12 to 16 days should pass, after which menstrual bleeding begins.

The duration of menstruation from 3 to 7 days, while the amount of released blood should not exceed 80 ml. If menstrual bleeding is longer, then this is a reason to go to a doctor. Also, it is necessary to talk about pathology, if previously the cycle was three days and the discharge was moderate, and then the menstrual periods began to go 6 days and the bleeding increased significantly. The menstrual cycle has the right to be unstable in the period of formation in adolescence and in menopause, when the reproductive function of a woman comes to an end.

Menstrual cycle and conception

Counting the menstrual cycle is important for all girls and sexually active women. For some, this is a way to get pregnant faster, for others - a way to protect against unwanted pregnancy. Favorable or dangerous days of the menstrual cycle (for those who do not want children at the moment) are the days of ovulation, which in a healthy woman fall in the middle of the cycle. If sexual intercourse was two days before ovulation or on the day of ovulation, then the probability of pregnancy is very high.

Sometimes pregnancy begins because of sexual intercourse, which was a week or more before ovulation. This happens for two reasons:

  • Spermatozoa males can remain motile in the female genital tract for up to 7 days, so conception occurs immediately after the release of the egg.
  • Ovulation occurred earlier than the approximate time. The date of ovulation to predict at 100% is always difficult.

Based on the above, the safe days of the menstrual cycle begin only a day after ovulation. By this time, the egg has already died, so fertilization is impossible. But you can only have unprotected sex life if ovulation was exactly (confirmed by measuring the basal temperature or using tests to determine ovulation). Sometimes the maturation of the egg occurs later than the expected period, so a free sex life in the second half of the menstrual cycle can also lead to pregnancy.

Violation of the menstrual cycle

A lifelong woman gets used to follow the menstrual cycle. This leads to the fact that small deviations from previous dates cause the appearance of panic. But not always change the duration of the cycle or the abundance of menstruation indicates the presence of disease. For example, the menstrual cycle after childbirth can not be called stable. Lack of menstruation, irregularity and a change in the nature of the discharge arise against the background of breastfeeding. Hormone prolactin, which is responsible for the full lactation, leads to a failure of menstruation, so you should not be afraid.

Many diseases of the sexual sphere can manifest changes in the cycle. The most common manifestation is considered algomenorrhea - it is pain during the menstrual cycle, which can be of varying intensity. This is usually a strong nagging pain in the lower abdomen, accompanied by nausea, vomiting, and headaches. A similar condition occurs with endometriosis. Weaker manifestations of algomenorrhea occur in women with inflammation of the uterus or appendages.

Dysmenorrhea is characterized by fluctuations in the duration of the cycle. In this case, menstruation may begin earlier or later than the expected period, that is, the duration of the menstrual cycle is always different. This usually occurs due to the lack of ovulation, so the first phase is stretched for an indefinite amount of time. Dysmenorrhea and pain during the menstrual cycle are sometimes combined together. This condition is called algodysmenorrhea, and it happens with frequent flights, polycystic ovaries, inflammation of the appendages and endometriosis.

Particularly should be allocated oligomenorrhea and amenorrhea. The first state is characterized by rare and slim menstruation, the other is characterized by the absence of menstruation for 6 months. In this case, the menstrual cycle is completely disturbed, since there are no cyclic changes in the uterus and in the ovaries, or they proceed very slowly. Therefore, in the absence of menstruation for a long time, one should not rejoice at this, but seek a doctor. Such a clinical picture occurs when infertility, as oligomenorrhea occurs when hormonal levels are disturbed.

How to treat disorders?

If the cycle has become unstable in duration, monthly abundant or scarce, then this is the reason for going to a gynecologist. Independently eliminate the delay of the menstrual cycle is almost impossible. Usually, such a condition is a sign of diseases of the female genital sphere, and treatment should be based on the exclusion of pathology, which led to cycle disorders.

The gynecologist prescribes an examination, which includes the detection of infection, gynecological ultrasound, if necessary, a blood test for female sex hormones. If this does not work, then the cause may be extragenital pathology (thyroid disease, adrenal glands, autoimmune diseases of the connective tissue, genital tuberculosis). How to restore the menstrual cycle in such cases? It is necessary to identify the disease and contact a specialist who deals with this pathology (endocrinologist, rheumatologist, tuberculosis specialist, etc.). Only a comprehensive joint treatment will help return the menstrual cycle to normal. Usually such situations are in women who are screened for infertility.

Knowledge of the regulation of the menstrual cycle, its duration and features will allow time to identify problems from the reproductive organs of women. This will help diagnose diseases such as uterine fibroids, endometriosis, polycystic ovaries. No disruption of the menstrual cycle should not be overlooked. Timely seeking medical help will prevent complications of the disease. Also, this knowledge is necessary for planning pregnancy. They will help you get pregnant much faster. A woman who understands the processes occurring in her body will be able to choose the right method of protection from unwanted pregnancy. Therefore, we should not neglect this information.

Cycle structure

The menstrual cycle consists of 4 periods. This time interval is counted from the first day of menstruation. The cycle ends immediately before the start of the new menstrual flow. Its standard duration is 28 days, but slight deviations are possible. The following phases are distinguished:

  • menstrual,
  • follicular
  • ovulation
  • luteal.

The names of the phases reflect the essence of the processes occurring in the body of a woman. However, a biphasic cycle is often considered. In this case, two of its longest stages are studied: follicular and luteal.

The time interval starting from the first day of menstruation and continuing until the moment of ovulation is called the phase of proliferation. The rest of the cycle is the secretion phase.

Changes occurring at different stages are controlled by the hormones of the hypothalamic-pituitary system. It is on these structures that the ovaries work.

The hypothalamus and pituitary control the phases of the menstrual cycle

Menstrual phase

The first stage of the cycle is characterized by the presence of bleeding of varying degrees of intensity. This phase lasts normally from 3 to 7 days. Total blood loss should not exceed 80 ml. Deviation from the specified data may indicate a malfunction of the reproductive system. The exceptions are periods of puberty and menopause.

The bleeding is a rejected endometrium - the inner uterine layer. In the previous cycle, it grew and compacted to allow the fertilized egg to consolidate within the uterus. In the event that pregnancy does not occur, the body gets rid of unnecessary endometrium. An unfertilized egg comes out with blood.

During this period, weakness and headaches may occur. The vast majority of women complain of pain in the lower abdomen.

The beginning of the cycle is considered the phase of monthly

The second phase of the menstrual cycle begins immediately after the cessation of bleeding. Sometimes menstruation is considered as part of the follicular phase. The countdown of the stage will start from the first day of discharge, and its average duration will be 14 days. The follicular phase corresponds to the first half of the cycle.

At this time, the woman's body starts preparing for the intended pregnancy. In the pituitary gland under the influence of the hypothalamus begins the active secretion of follicle-stimulating hormone. This process triggers the production of estrogen by the ovaries.

Thanks to these hormones, the follicles - the structural units of the ovaries - begin to grow rapidly. In one of them the egg matures. The follicular phase is characterized by a gradual thickening of the endometrium. Forms a favorable environment for implantation of a fertilized egg.

The concentration of estrogen increases throughout the entire follicular phase, reaching its peak on the last day before ovulation. This phenomenon is inextricably linked with the maximum secretion of pituitary luteinizing hormone.

External manifestations of hormonal fluctuations during the follicular stage range from excessive irritability of a woman at the beginning to an emotional lift at the end.

The follicular phase is characterized by enhanced follicular growth in the ovaries.

Approximately in the middle of the menstrual cycle, ovulation occurs. This is the process of release of ripe egg. The follicle shell breaks, and the oocyte begins to slowly move through the fallopian tubes. The ovulation phase is the shortest. Its duration is equal to two days, and the average life of the egg does not exceed one day.

From this phase of the menstrual cycle, the intensive secretion of estrogen by the ovaries stops. The main effect on the woman's body begins to have another hormone - progesterone.

The process of release of the egg is controlled by the pituitary gland. On day 14 of the cycle, the secretion of luteinizing and follicle-stimulating hormones is most intense. This phenomenon determines the maximum chances of a woman to become pregnant.

It was at this time that women increase libido. Vaginal secret acquires a slimy character. In addition, there may be a slight bleeding, which is accompanied by mild soreness in the abdomen. This is the norm. Another external manifestation of the ovulation phase is a slight increase in body temperature.

Ovulation phase - optimal time for conception

Luteal phase

If a mature egg has not been fertilized, the time comes for the next phase of the menstrual cycle. The duration of the luteal phase in women varies. The longer this stage, the greater the number of days included in the full cycle.

In place of the bursting follicle a yellow body forms. The secretion of luteinizing hormone by the pituitary gland continues. It triggers progesterone production by the ovaries. The concentration of the latter increases sharply immediately after ovulation and remains consistently high until the beginning of the menstrual flow in the next cycle.

Due to the effect of progesterone, the endometrium becomes friable. In pregnancy, this allows easy implantation of the embryo in its walls. At the end of the luteal phase, the endometrium reaches its maximum thickness. He will begin to reject from the first day of the next cycle.

A woman under the influence of progesterone becomes irritable. The deterioration of mood and well-being reaches its peak at the very end of the luteal phase. This condition is called premenstrual syndrome. Other symptoms of menstruation may be observed:

  • breast swelling,
  • swelling of the face and limbs
  • weakness,
  • increased fatigue
  • itchy skin
  • acne,
  • nausea,
  • dizziness.

The intensity of the symptoms is an individual indicator.

It is the current phase of the menstrual cycle that determines the likelihood of pregnancy. In addition, the hormones produced by the ovaries, affect the appearance of a woman, her emotional state and well-being. It should be remembered that the main condition for planning a pregnancy is cycle stability. For any changes in one of its stages, it is required to consult a gynecologist-endocrinologist.

The phases of the menstrual cycle are of great importance not only in the planning of pregnancy, since their proper alternation characterizes the entire female reproductive system. Знание этих фаз по дням, позволяет более подробнее разбираться в потребностях организма женщины и исходя из этого планировать свои действия.

Во время каждой менструации в организме периодически происходят значительные изменения, которые предназначены природой для зарождения, сохранения и развития новой жизни. Normally, the cycle is 28 days, with small deviations in one direction or another. Even with the same woman with a well-established cycle, its duration can sometimes vary for various reasons. But these fluctuations should not occur often and not exceed 3-4 days. At this time, continuous complex processes occur in the ovaries, which are characterized by the appearance of a new state — either pregnancy, or the expectation of the next menstruation. The beginning of the cycle is considered to be the beginning of menstruation, that is, its first day, and the end is considered to be the last day before the next menstrual periods.

Phases of the menstrual cycle by day are as follows.

Follicular phase

The cycle originates from the follicular phase, lasting on average about 14 days. In the first 4 to 5 days, menstruation passes, after which the body begins preparing for a possible pregnancy under the influence of follicle-stimulating hormone (FSH), produced by the pituitary gland. Follicles develop in the ovaries, the shell of which produces estrogen. Several follicles can simultaneously develop in them, but under the influence of luteinizing hormone (LH) and follicle-stimulating hormone, only one of them reaches the dominant size and can continue to develop. The rest are regressing. In addition to the development of the follicle, during this period, a detached layer of the endometrium is removed from the uterus, which is excreted with secretions, that is, menstruation occurs. In the uterus, a new layer of the endometrium is building up, designed to receive a fertilized egg.

Ovulatory phase

From the 15th day, the ovulatory phase of the menstruation cycle begins, by days it is considered the shortest. The existence of an egg cell capable of fertilization is small — only a day, but according to some sources it can last up to 2 days. At this time, an increase in the level of the hormone LH is observed, and the concentration of FSH, on the contrary, decreases. But it is at this time that the egg is ready for fertilization. It moves along the tube with the help of epithelial villi. Further fate offers two options: fertilization and further development, or without waiting for fertilization, death and dissolution in the endometrial mucosa.

Despite the allotted short time, to track the exact time of the beginning of the ovulatory phase can be on elevated basal temperature. With anovascular cycle, this phase does not occur. The follicle in this case develops and matures, but does not burst, but dissolves.


  • Do you want to conceive a child for a long time?
  • Many ways tried, but nothing helps.
  • Diagnosed thin endometrium.
  • In addition, the recommended medications are for some reason not effective in your case.
  • And now you are ready to take advantage of any opportunity that will give you a long-awaited baby!

Phases of the menstruation cycle - the time interval when there are transformations in the female body, aimed at conception, the onset of pregnancy, and in the absence of fertilization at the beginning of menstruation. The entire menstrual cycle lasts no longer than 35 days, the shortest is 21 days. The ideal option is the duration of 28 days. Due to the fact that during the month the female body is affected by multiple factors, the duration of the second phase of the menstrual cycle may be reduced or reduced. Normal is considered a deviation of not more than 7 days in one direction or the other. What happens in the female body during the month, how to calculate the phases of the menstrual cycle?

The onset of the first part of the menstrual cycle falls on the beginning of menstruation. On the same day, hormonal changes. The main thing in this period is the follicle-stimulating hormone - FSH. He starts the process of developing a new egg. 5–7 follicles appear. But one of them in 11–16 days reaches the size of 14 mm, stands out among the rest. This follicle, in which the egg cell has matured, is ready to go outside. In addition, under the influence of hormones, the uterus is cleared from the endometrial layer, the growth of the new begins. The sexual organ is re-preparing for the adoption of a fertilized egg.

The duration of the first phase of the menstruation cycle in the absence of pathologies, diseases of the reproductive system is 7-22 days. The best option is when all necessary conversions are completed 10–15 days from the start of the cycle. In general, the duration of the entire menstrual cycle depends on the time of maturation of the egg. It ends with the onset of ovulation. However, 2 times a year, no ovulation is allowed. Then the cycle is called anovulatory.

During the first phase, the endometrium layer is cleared, the growth of the new, the maturation of the follicle with the egg cell occurs.

How to count monthly cycle

To understand how to count the cycle of menstruation, you must clearly understand what it is. Women think that they know everything about menstruation, and the phrase monthly cycle is familiar to them, but many, especially the young, have no idea how to calculate the menstrual cycle.

In a woman's body, or rather, in its reproductive system, regular changes occur, and the period between the months from the beginning of one to the start of the next menstruation is called the menstrual cycle.

It is blood discharge, and not a scanty brown daub, and is the first day of the cycle, from which you need to count the days until the first day of blood discharge during the next menstruation.

Now I understand how to calculate the cycle of menstruation. However, we still have to figure out which cycle of menstruation is considered the norm. That is, it is necessary to determine its optimal length.

The duration of menstruation is usually 3-7 days, and the cycle is considered optimal when its length is 28 days. However, deviations up to 5-7 days, if they are regular, belong to the norm. But when the cycle is less than 21 or more than 35 days, it is necessary to find out the reason by contacting a doctor.

There are exceptions when violations are considered the norm:

  • its installation in adolescents (can last up to 2 years),
  • after childbirth, as well as breastfeeding,
  • hormonal disorders,
  • menopause,
  • taking drugs.

All other causes are caused by pathology, you need to consult a doctor.

What should be considered a monthly cycle

There are 4 reasons to count the menstrual cycle:

  • to exclude pregnancy
  • to get a chance to get pregnant,
  • to maintain proper hygiene,
  • in time to identify emerging diseases.

Knowledge of how to count the days to determine ovulation, tell a woman how many days before her appearance, it is necessary to begin sex life. Only then you can count on conception.

Women who do not plan pregnancy, on the contrary, it will serve as a natural method of contraception. They will be able to correctly and accurately calculate the days called dangerous.

A woman is armed if she knows when to expect critical days, which means that the bleeding will not take her by surprise. By the cycle, or rather, its regularity or irregularity, it is easy to notice a possible problem, which will help to consult a doctor in time.

Menstrual calendar

Every woman is unique, therefore all calculations should be carried out according to her individual data. It is difficult to remember which day menstruation started 2, 3 or 4 months ago. And to know this is important. There is an old and reliable way that our grandmothers used. Need to keep a calendar. To do this, you need to prepare a calendar, pen and notebook.

The first day of menstruation should be circled and repeated every month. The appearance in the calendar of 4-5 such circles makes it possible to make an analysis and calculate everything correctly. For example, the first day of menstruation fell on September 8, and in October on the 5th, then the cycle was 28 days. This value should be constant monthly.

Sometimes it can fail, which can increase or decrease the cycle, for example, by 3 days. Such a slight change is considered normal, but it is necessary to analyze your condition, perhaps it is caused by stress, climate change or medication. If the failure occurred again, and there was no explanation, you should consult a doctor.

How to determine favorable days for conception

Calculate suitable for conceiving days will help marked calendar, for example, for 6 months. We need to take it and determine the shortest and longest cycle. For example, it will be, respectively, 27 and 30 days. 18 days should be subtracted from the short, and 11. should be taken from the long one. We obtain the following calculations:

So determine the favorable period for conception. In the specific case it will be an interval from 9 to 19 days.

The date of ovulation can be correctly calculated in another way. Here again need a calendar. Consider this example: the duration of the menstrual periods for 6 months is known, you need to summarize them. For example, 27 + 29 + 28 + 27 + 29 + 28 = 168. Next, the length of the average monthly cycle should be calculated: 168/6 = 28, and then it should be divided by 2. We get 14. This is the day of ovulation, we add to it 5 days in both directions 14-5 = 9 and 14 + 5 = 19. From 9 to 19 days will come the days that are favorable for conception.

All others will be unfavorable in relation to pregnancy. These are periods from 1 to 9 and from 19 to 28 days.

Nowadays, special programs have been developed to help women, which can be installed on a mobile phone. Just enter the start date and the average length of the menstrual period, and the days favorable for conception will appear on the screen.

The phases of menstruation and hormonal background

The menstrual cycle is divided into three phases: follicular, ovulation and luteal (phase of the yellow body).

At this stage of the menstrual cycle, follicles develop in the ovaries. The process is controlled by follicle-stimulating hormone (FSH), luteinizing (LH) and estradiol. The level of hormones is growing. The endometrium is in the proliferative phase.

During this phase, the mature egg leaves the bursting follicle, first entering the abdominal cavity and then into the fallopian tubes. This occurs at the peak of the LH surge. Then its level begins to decrease. The endometrium at this time is preparing to receive a fertilized egg (secretory phase). Fertilization usually occurs in the fallopian tubes.

In place of a bursting follicle in the ovary, a so-called yellow body is formed. The level of LH and estrogen decreases, the level of the pregnancy hormone progesterone increases. This phase continues until the onset of menstruation.

If fertilization does not occur, during menstruation, the functional layer of the endometrium is rejected. With menstruation, a woman may experience physical and emotional discomfort. One of the symptoms is nausea during menstruation. If a woman noticed the so-called black periods, she should consult a doctor.

Various diseases during menstruation are also possible. For example, cystitis in menstruation - inflammation of the bladder. If a woman notices that the disease occurs during menstruation, you should immediately contact a gynecologist.

As for the possibility of becoming pregnant, it is at almost every moment of the cycle. Do not hope that during menstruation you can not get pregnant. As the practice of gynecologists proves, women may well become pregnant during menstruation.

First menstrual phase

The beginning of the first menstrual phase can be considered the first day of the menstrual cycle. For the first phases of the menstrual cycle often characterized by painful menstruation - algomenorrhea. As a rule, the painful sensations are caused by inflammatory and infectious diseases of the small pelvis. Therefore, it is better to cure these diseases once and forget about the pain, than to suffer each month and put your health at risk.

First menstrual phase Doctors recommend that women avoid any physical exertion, rest more, use foods with a high content of iron.

On average, the first menstrual phases last from 3 to 6 days.

Second menstrual phase

The second phases of the menstrual cycle continue up to two weeks after the end of menstruation. Under the influence of signals from the brain, which sends impulses, the follicle-stimulating hormone enters the ovaries of a woman, which promotes the development of follicles. After a certain period of time, a dominant follicle is formed, in which all the stages of maturation of the egg cell will subsequently pass.
The second phase of the menstrual cycle is characterized by the release of the hormone estrogen, which renews the lining of the uterus. This estrogen affects the cervical mucus, as a result of which it becomes immune to the effects of sperm.

Often, stresses and various diseases contribute to an increase in the duration of the second phase of the menstrual cycle, thereby delaying the onset of the third. At the same time, the possibility of what is often called "getting pregnant after menstruation."

Andrey Stepanov - doctor, kmn

The average length of the menstrual cycle is 29 days, so menstruation is traditionally also called monthly. Obviously, this term was formed historically due to the simple coincidence of the duration of biological and astronomical rhythms. However, in 1986, it was demonstrated that in 28% of women ovulation occurred during periods of full moon. In the other 7 phases of the lunar cycle, only 8-12% of women ovulated each.

After analyzing the change in the concentration of melatonin in the blood of women, the authors concluded that a decrease in the photosensitive hormone in the full moon unblocked the synthesis of estrogen and initiated ovulation. But how can the menstrual cycles of a person’s daily life can be related to the phases of the moon? The answer to this question can be found in the distant past of the ancestors of mammals, which were once exclusively nocturnal animals!

The first mammals were nocturnal animals.
Appearing 230 million years ago, the first mammals for another 160 million years were living with dinosaurs. In those days, large predatory dinosaurs still dominated the planet, but not 24 hours a day. As night fell, as a result of lowering the ambient temperature, the metabolism of cold-blooded dinosaurs slowed down. Meanwhile beast-like lizards (genus. Synapses), in order not to compete for a niche with larger dinosaurs, began to adapt to the activity at night and acquired the ability to maintain a high body temperature regardless of the ambient temperature and for the first time acquired hair.

But as a result, the smallest herbivorous synapses survived, capable of quickly warming up their small body at night, and hiding in holes during the daytime in order not to catch the eye of large predatory dinosaurs. Thus, adaptation to unique conditions has turned large, cold and bald reptiles into small fluffy warm-blooded animals, resembling modern squirrels:

These evolutionary acquisitions will repeatedly be useful to mammals on the path to dominance on the planet, but for now we will pay attention to the changes related to their adaptation to life in the twilight and lunar cycles.

Adaptation of the organ of vision to nocturnal lifestyle
For maximum photosensitivity at night, the organ of vision of the first mammals gradually changed towards increasing the area of ​​the cornea. In addition, in addition to cone photoreceptors in the eyes of night mammals, highly sensitive photoreceptor rods appeared in evolution for the first time in evolution, which provide the ability to see at dusk. Also, additional photosensitivity was made possible by increasing the number of ganglion cells located in the surface layer of the retina. This arrangement provides these photoreceptors with a higher photosensitivity than that of rods and cones that are at the very bottom of the retina:

Influence of photoperiods of the Sun and the Moon on biorhythms
In contrast to the rods and cones that provide vision, ganglion cells are responsible for transmitting information about the degree of light to the body. Even minimal stimulation with light through closed eyelids is enough to activate ganglion cells and transmit the signal along the optic nerve to the hypothalamus. As a result, the impulses of the hypothalamus are blocked in the pineal gland (epiphysis), which in response to this stops synthesizing melatonin:

In turn, a decrease in melatonin concentration unlocks the synthesis of pituitary hormones, resulting in activation of the sex glands (germ cell maturation and sex hormone synthesis), the thyroid gland (accelerating metabolism and increasing body temperature) and adrenal glands (mobilization of carbohydrates and stress resistance). Under the condition of an arbitrary awakening after dawn, we can observe the effects of such hormonal switching every morning in the form of high physical and sexual readiness, good appetite, vascular and immunological tolerance to cold weather, etc. For example, I noticeably wake up more easily after dawn in spring and summer and with difficulty until dawn in late autumn and winter. Thus, the Sun determines the daily biorhythm of modern animals and humans, and also affects their seasonal activity.

Аналогичным образом происходили переключения режимов активности и в организме первых млекопитающих, но в следствии ночного образа жизни оптическим осциллятором их биоритмов являлась Луна с принципиально иными фотопериодами нежели у Солнца. В полнолуние отражаемый Луной солнечный свет имеет интенсивность всего 0,3-1 люкс, но этого более чем достаточно для влияния на биоритмы ночных животных. This light blocked melatonin synthesis, with the result that metabolic, physical and sexual activity increased. Thus, in females of the first mammals, the maturation and ovulation of the egg presumably coincided with the full moon period, at the same time the spermatogenesis and sexual activity of the males were activated. In addition, a higher illumination in the full moon contributed to the search for food and activity in general. But, as we now know, it was not forever.

The transition of mammals to daytime life
About 65 million years ago, as a result of the global cooling, large cold-blooded dinosaurs became extinct, giving up the daytime niche to warm-blooded mammals. The transition of the majority of mammals to the daytime lifestyle provoked a stormy speciation and accelerated their evolution. Including due to the warm-bloodedness, mammals safely populated all the environments of the biosphere - the dungeon, the land, the water, the atmosphere and even space.

The biorhythms of daytime mammals have since started to be set mainly by the Sun, and the periodicity of reproductive activity has changed significantly in the vast majority of species. For example, in many wild mammals, ovulation occurs only once a year in the spring as the day length increases, indicating that the leading role of melatonin in the regulation of sexual activity remains. In general, the frequency and duration of sexual cycles in modern mammals varies widely. Including among primates, the duration of the menstrual cycle is not uniform, but in 80% of primate species, this indicator is focused in the range of 20-40 days.

Why does a man’s menstrual cycle retain not only its high-frequency, but also its duration, approximately equal to the lunar month, as well as synchronism with the phases of the earth satellite? It is possible that this synchronization contributes to reproductive activity, and the species and individuals that preserve it have an evolutionary advantage. As a result, despite the low reproduction rate (one pregnancy = one fetus), but also thanks to year-round fertility and high sexual activity, the person safely settled the entire planet, setting a record number (the average population of animals similar in body weight is 100,000) .

There are a number of studies in favor of this thesis. First, it was demonstrated that the closer the duration of the menstrual cycle to 29.5 ± 1 day, the higher the fertility of the woman. In other studies, it was shown that women with a long menstrual cycle of 30 ± 3 days are sexually more active, they have sex before and later menopause. Another study allowed to establish that in women with a menstrual cycle of 29.5 ± 1 day, the synchronization of the stages of the cycle with the phases of the moon is more often observed.

The above stated would be more convincing if the person led a nocturnal lifestyle without using artificial light, the intensity of which is several orders of magnitude higher than the light reflected by the Moon. However, we are day creatures, and even those of us who are owls see the moon except on weather widgets.

How to explain the preservation of this biorhythm after millions of years of evolution in the conditions of daytime lifestyle and the use of artificial lighting? It is obvious that in the neuroendocrine system there must be another - non-optical connection with the Moon.

Gravity Sensors Hypothesis
It is traditionally considered that the gravitational influence of the Moon on living organisms is mediated solely by the ebb and flow of the world's oceans. The tidal amplitude is maximal during the periods of the new moon and the full moon, when the sun and the moon are located opposite the earth and their gravitational forces are summed up (1 and 2 in the diagram below). During the phases of a rising and decreasing Moon, the tidal forces of the Moon and the Sun directed at right angles partially level each other, therefore the tides at these time periods are noticeably weaker (3).

Thus, during periods of full moon and new moon, the summed gravity of the Moon and the Sun indirectly (for example, changing access to food) affects living organisms living in the literal zone (land area exposed during periods of ebb tide). Obviously, in the case of primates, the evolution of which took place mainly in the interior of the continent, the influence of gravity on biorhythms cannot be justified by ebbs and flows.

Nevertheless, the hypothesis of gravitational effects on the human endocrine system is indirectly confirmed in studies. In 2013, Cell magazine published the results of a double-blind study of volunteers who spent the night in a room isolated from light. The concentration of melatonin, the speed of sleep and the duration of sleep were significantly higher during the periods of the new moon, when the gravitational forces of the Sun and the Moon are unidirectional and therefore summed. In another study, the frequency of labor increased during the period of minimal gravity of the moon (the apogee of the lunar orbit), a similar relationship was shown in the study of the frequency of miscarriages. It is also interesting that, according to some authors, women ovulate more often at the new moon, and according to others at the full moon, that is, during periods of maximum gravitational effects. However, the authors find it difficult to explain the mechanism for implementing the detected dependencies.

Thus, there is evidence in favor of the presence of the gravitational field of the Moon on humans, which is implemented directly, and not indirectly by ebbs and flows. In this case, a sensor with sufficient sensitivity to a relatively weak gravitational field should be present in the body.

It is known that the vestibular apparatus (the organ of equilibrium) transmits to the central nervous system information about the acceleration of the body and its position relative to the forces of the gravitational field of the Earth, but its sensitivity is sufficient only to perceive high-amplitude oscillations with a frequency of about 1 Hz. Meanwhile, the amplitude and frequency of oscillations of the gravitational forces of the Sun and the Moon near the Earth's surface are several orders of magnitude lower than these values. Nevertheless, the device and the principle of action of the vestibular apparatus prompted researchers to search for a similar hypothetical gravity sensor in the endocrine system.

In 2005, the well-known biologist Alexei Olovnikov was the first to hypothesize gravity sensors, according to which the mediators of energy transfer of gravity may be crystalline inclusions of calcium compounds (rounded calcifications — spherulites), which are actually found in the epiphysis of all healthy people. In the vestibular apparatus, similar formations (ear stone - otolith) are just responsible for sensing the position and acceleration of the body in space. Thus, in the vestibular apparatus and in the epiphysis, similar formations are found, which at least in the first organ are unambiguously responsible for the perception of gravity. However, the most interesting is the proposed mechanism of the work of the hypothetical gravisensor.

In the case of the vestibular apparatus, with a change in the position of the head in space, the otoliths are displaced and exert pressure on the processes of the sensory cells, which in response to this are activated and transmit a signal to the central nervous system. In the case of the epiphysis during periods of full moon, there is a shift of spherulites from the center of the cell to its processes - directly to the membrane. Here, spherulites in conditions of lower pH release calcium ions, the concentration of which determines the activity of melatonin secretion in the blood. This explains the higher amplitude of melatonin concentration during periods of increasing gravity. In addition to the analogies between the vestibular apparatus and the epiphysis, differences in the periods of formation and sizes of calcifications in these organs are no less informative.

In the case of the vestibular apparatus, the otoliths are still in utero, reaching sizes of 3-6 microns, and the epiphysis spherulites begin to be detected only on the eve of puberty and gradually reach 47 ± 28 microns:

The larger sizes of the epiphysis spherulites are regular: the larger the size and / or density of calcification, the higher its sensitivity to gravity, which is important for assessing the weak gravitational field of the Moon and the Sun. As the size of spherulites increases, they merge with each other to form aggregates up to 1 mm in size:

These aggregates gradually crowd out the endocrine cells of the pineal gland, which leads to a decrease in melatonin secretion with age. Why is this so important? The fact is that high round-the-clock concentration of melatonin in children inhibits puberty, and a decrease in the daily amplitude of its secretion removes this inhibition and starts puberty. Thus, according to the hypothesis, calcification presumably leads, on the one hand, to a decrease in melatonin secretion and puberty, and, on the other hand, to an increase in the amplitude of the hormone during full moon and / or new moon periods, which causes the appearance of monthly hormonal cycles not only in women, but also in men.

The gravitational hypothesis is extremely brave and controversial, but I want to remind you that Olovnikov is the author telomeric hypothesis cellular aging, which was proposed to them in 1971. Then his colleagues laughed at him, but in 1998 the existence of telomeres was experimentally proved by American scientists, for which the latter received the Nobel Prize in 2009. This I mean that the hypothesis of gravity sensors belongs to a reputable author. Maybe someone is already waiting for the next Nobel Prize for the discovery of the mechanism of biological clocks or even the methods of reversing the aging process!

In the future, I plan to open the topic of biorhythms more widely in the context of the problems of slowing down the aging process and preserving youth. In the meantime, I will continue the topic of menstrual cycles and in subsequent articles I will talk about the causes of deterioration in mood with PMS and the causes of the phenomenon of menstrual bleeding. In order not to miss - subscribe to the most popular blog about medicine in Russia! And if you do not have an account in LiveJournal, subscribe to updates in Facebook, Vkontakte, Telegram and Twitter.

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