Night shift work is linked to menstrual irregularity and endometriosis


According to a study being presented at the 23rd European Congress of Endocrinology (e-ECE 2021) on Sunday 23 May, women working night shifts may be at a greater risk of menstrual irregularity and developing endometriosis.

The research found a reduction in the expression of PER-2, CRY-1 and CLOCK genes along with an increase in REV-ERBb in ectopic compared to eutopic tissues. Prior to this research, there had been no previously published studies relating to the alterations in core clock-genes and the impact on women with endometriosis.

Endometriosis is a condition in which tissue similar to the lining of the womb starts to grow in other places, such as ovaries and fallopian tubes. Endometriosis affects roughly 10% (190 million) of reproductive-age women and girls globally. The symptoms of endometriosis can vary – some women are badly affected, while others might not have any noticeable symptoms.

In severe cases, it can be very painful and can cause infertility, miscarriages and ectopic pregnancies due to the probable effects of endometriosis on the pelvic cavity, ovaries, fallopian tubes, or uterus.2

Disruption of circadian rhythm in night shift workers has been associated with menstrual irregularity, as well as an increased chance of developing endometriosis and ovarian tumours.

Dr. Narjes Nasiri-Ansari, Dr. Aggeliki Karapanagioti, and a team of colleagues under the guidance and supervision of Professor Eva Kassi from the National and Kapodistrian University of Athens, Greece, investigated the expression of the core clock-related genes in paired eutopic and ectopic endometrial tissues.

The study looked at 27 patients with confirmed ovarian endometriosis. Eleven (11) paired samples were collected from ovarian cysts (ectopic endometrial tissues) and normal endometrium (eutopic tissues), while a further eight (8) ectopic and eight (8) eutopic endometrial tissues were collected from 16 different patients with the same diagnosis.

“The clinical evidence that circadian rhythm disruptions can be associated with endometriosis is now confirmed at tissue level by the altered expression of local clock genes in ectopic endometrium. Understanding the causes and effects of endometriosis will improve our ability to detect, manage or even prevent the condition. These findings provide us with a better understanding of biological rhythm disturbances,” commented Professor Eva Kassi.

The results from this study demonstrate an altered expression of CLOCK, CRY1, PER-2 and Rev-ERBb in normal endometrium tissues, as compared to ectopic endometrial tissues, indicating a disturbance of biological timing. However, the causal relationship of the altered expression pattern of these genes with the development of endometriosis needs further investigation.

The menstrual cycles is controlled by the cyclical secretion of reproductive hormones, including luteinizing hormone, follicle-stimulating hormone, estrogen, and progesterone, which are regulated by the hypothalamus – pituitary – ovarian axis.1 The absence of menses ≥3 months is diagnosed as secondary amenorrhea.

Over 50% of secondary amenorrhea cases were due to a disturbance of hypothalamic-pituitary-adrenal axis, which can lead to infertility.2 In addition, estrogen deficiency caused by secondary amenorrhea increases the risk of cardiovascular disease, osteopenia, and depression.3-5

Menstrual cycles are assessed by their length and regularity and are considered as a marker of reproductive health.6, 7 Disruption of the circadian rhythm during shift work affects the regulation of the hypothalamus-pituitary-ovarian axis and alters the menstrual cycles.8, 9

Studies using data from the Nurse’s Health Study II (71 077 female nurses) and Nurse’s Health Study III (6309 female nurses) reported that the incidence of irregular menstrual cycles were 11%-19%, and that the age-adjusted prevalence risk (PR) was 1.20-1.34 for irregular menstrual cycles in women working rotating shift.8, 9

The Nurse’s Health Study II showed that women of Asian ethnicity had a higher risk of irregular menstrual cycles than that of white people (PR: 1.38, 95% confidence interval [CI]: 1.19-1.61).8 Polycystic ovary syndrome (PCOS) is a common endocrine disorder in women of reproductive age and irregular menstruation, and the polycystic ovary subgroup is relatively common in Asian ethnicities.10

In addition, genetic studies on PCOS reported that there is polymorphism in follicular stimulating hormone receptor gene, which plays key role in a menstrual cycle.11, 12 These might be the reasons for the higher risk of irregular menstrual cycles in Asian ethnicities.

The prevalence of dysmenorrhea and premenstrual symptoms exceed 70%, and dysmenorrhea can deteriorate women’s quality of life.13, 14 However, the effect of rotating shifts on dysmenorrhea is still controversial. While the study on 113 women working rotating shift and 75 women working day shift reported that rotating shift was not associated with dysmenorrhea,15 a study on 420 Taiwanese nurses showed that a higher percentage of women with dysmenorrhea worked under three shift rotations than women without dysmenorrhea (91.3% vs 82.9%, P = .014).14

In each country, working hours and conditions vary. While night shift working hours are regulated within 12 hours in many counties, >90% of hospitals in Japan adopt a 16-hour night shift as two rotating shifts.16 While a 16-hour night shift may increase the risk of irregular menstrual cycle compared with a 12-hour night shift; however, data are scarce regarding the incidence of irregular menstrual cycle among Japanese nurses.

Differences in ethnicities and working conditions could affect the development of irregular menstrual cycles. Therefore, the present study aimed to determine the prevalence of irregular menstrual cycles and dysmenorrhea in female nurses working under rotating shifts in Japan and to evaluate the effect of night shifts on irregular menstrual cycles and dysmenorrhea.

reference link:

More information: Abstract 1394: Alterations in clock genes expression in Eutopic and Ectopic Endometrial Tissue,


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