How Night Shifts and Jet Lag Impact Female Hormones and Biological Clocks
A female flight attendant soaring across continents and a female mariner living at sea for months at a time work in entirely different environments—the skies and the oceans. Yet, their professions share a striking commonality: experiencing a normal life of waking with the sunrise and sleeping after sunset is nearly impossible.
Flying through the night and experiencing shifts in day and night within hours, combined with repetitive night watches and shift rotations aboard ships, places immense strain on the body. This raises a fundamental question: what happens to the bodies of women who live outside of “earthly time” for extended periods?
The Brain-Gonad Axis and Circadian Rhythms
Recent medical science focuses less on the uterus itself and more on the biological clocks residing within the body. Female menstruation and ovulation are not governed by the uterus and ovaries alone. The hypothalamus in the brain, the pituitary gland, and the ovaries constantly signal each other to create a precise rhythm of reproductive hormones. When day and night are repeatedly inverted and sleep schedules become irregular, this biological rhythm inevitably faces disruption.
A domestic cohort study tracking 8 years of data, published in the international journal Occupational Medicine, investigated the connection between night work and irregular menstrual cycles. The researchers tracked how long-term exposure to night shifts relates to changes in women’s menstrual cycles, treating night work as a significant occupational exposure factor rather than a mere lifestyle habit.
Similar signals emerged in a systematic literature review analyzing 16 cohorts involving 123,403 women. Shift-working women experienced menstrual disorders more frequently than day-shift workers, and night work correlated with an increased risk of early spontaneous abortion. However, the researchers noted that once confounding variables like age and lifestyle were adjusted, the relationship with infertility was not entirely clear, drawing the line that evidence is not yet robust enough to restrict shift work for women of childbearing age simply on this basis.
Why Night Work Affects Menstruation
Our bodies operate on a circadian rhythm governed by an internal clock spanning roughly 24 hours. Not only sleep-wake cycles, but body temperature and the secretion of various hormones—including cortisol and melatonin—align with this clock.
Female reproductive functions are independent of this system. Exposure to bright light at night, sleeping during the day, and chronic sleep deprivation can destabilize the signaling pathways connecting the biological clock to reproductive hormones.
The Unique Stresses of Flight Attendants: Jet Lag and Cosmic Radiation
Flight attendants represent one of the most extreme examples of this disruption.
They are not ordinary night-shift workers. Traveling from Seoul to Europe or the Americas within hours creates a stark mismatch between the body’s internal clock and local time. Continuing to fly before recovering leaves no opportunity for the biological clock to adapt to a new time zone.
The U.S. National Institute for Occupational Safety and Health (NIOSH) explains that pilots and flight attendants experience circadian rhythm disruptions due to time zone shifts and night work, making this an occupational factor that must be considered in reproductive health.
A further unique variable is cosmic radiation.
Higher altitudes increase exposure to ionizing radiation originating from space compared to individuals on the ground. Consequently, NIOSH classifies pilots and flight attendants as groups occupationally exposed to cosmic radiation, with exposure levels rising particularly on long-haul, high-latitude, and polar routes.
Studies focusing on pregnant flight attendants have yielded noteworthy insights.
A team of U.S. researchers analyzed 958 pregnancies among 764 female flight attendants alongside roughly 2 million actual flight records.
Flight attendants who flew for 15 hours or more during early pregnancy during times they would normally be sleeping faced about a 1.5 times higher risk of first-trimester miscarriage compared to those who did not. Elevated miscarriage risks were also observed among those facing heavy physical workloads, such as standing or walking for long periods, or repeatedly bending at the waist.
Signals also appeared regarding cosmic radiation. The study observed that cosmic radiation exposure exceeding certain thresholds during early pregnancy could correlate with an increased risk of miscarriage between weeks 9 and 13.
Nevertheless, both the researchers and U.S. public health authorities emphasize that this should not be interpreted as a direct cause-and-effect relationship meaning “flying causes miscarriages,” as countless variables—such as individual actual exposure, flight schedules, age, and underlying health status—are involved.
Female Mariners: Navigating Isolation and Shift Work
What about women at sea?
Female mariners likewise live away from land for extended periods, but their environmental exposures differ from those of flight attendants. High-altitude cosmic radiation is not the core issue.
Instead, the crucial factors involve living in confined spaces for months, enduring repetitive night watches, shift rotations, sleep deprivation, fatigue, and stress.
Therefore, when explaining reproductive health issues in female mariners, approaching it from the perspective of “how long-term shift work, irregular sleep, and stress impact female biological rhythms” proves more accurate than blaming the sea itself. Large-scale studies on female workers have similarly reported higher rates of menstrual irregularities and certain reproductive health issues among shift- and night-work groups.
Distinguishing Pathologies from Rhythm Disruption
One important distinction must be made here.
There is currently insufficient evidence to conclude that flight attendants or female mariners are uniquely prone to uterine myomas (fibroids), adenomyosis, endometriosis, or ovarian cysts.
Such conditions are governed by complex factors, including age, genetics, hormones, and parity. A single profession should not be singled out as the direct cause.
Conversely, “time-sensitive reproductive functions”—such as shifts in menstrual cycles, ovulation, and pregnancy maintenance—tell a different story.
From a fertility medicine standpoint, female ovulation is not decided by the ovaries alone. Signals originating in the brain’s hypothalamus travel through the pituitary gland to the ovaries, and hormones like estrogen and progesterone subsequently feed back to the brain and uterus. This constitutes the hypothalamic-pituitary-ovarian (HPO) axis. Sleep, light, stress, and circadian rhythms are inextricably linked to this system.
Thus, if menstrual cycles suddenly shorten or lengthen, or irregular periods persist for months, dismissing it simply as “due to the job” is unwise. Hidden causes, such as thyroid disorders, polycystic ovary syndrome (PCOS), hyperprolactinemia, weight fluctuations, or diminished ovarian reserve, could be at play.
Particularly for women preparing for pregnancy, tracking menstrual cycles is vital. If cycles fluctuate significantly, amenorrhea occurs, or pregnancy proves difficult, receiving a basic evaluation at an obstetrics and gynecology clinic regarding ovulation status, ovarian reserve, and thyroid function is advisable.
NIOSH now manages the reproductive health issues of flight attendants and pilots as a distinct occupational health domain. Recommendations suggest reducing physical demands like prolonged standing, heavy lifting, and repetitive bending for pregnant flight attendants, alongside adjusting work schedules to minimize exposure to cosmic radiation and circadian disruption whenever possible.
Conclusion
Ultimately, the issue lies neither in the skies nor the oceans.
The human body evolved over millions of years to align with Earth’s day and night. Amid rhythms of moving when the sun rises and resting when darkness falls, the brain, ovaries, and uterus synchronized their own internal times.
Night shifts, long-haul flights, time zone changes, and shift rotations created by modern society constantly demand new schedules from that ancient biological clock.
The question posed to us by women flying through the skies and navigating vast oceans is simple. We have gained the ability to live independently of geography, but have our internal biological clocks truly freed themselves from Earth’s day and night? The answer provided by medical research to date leans heavily toward “not yet.”
※ This article was synthesized by combining a meta-analysis on shift work and women’s menstruation published in the international journal SSM – Population Health, an 8-year cohort study in Occupational Medicine, and reproductive health studies and guidelines for flight attendants from the U.S. National Institute for Occupational Safety and Health (NIOSH) and the Centers for Disease Control and Prevention (CDC). Key references include findings on the links between circadian rhythm disruption from night/shift work and menstrual irregularities, dysmenorrhea, and early menopause, as well as risks of pregnancy and miscarriage linked to time zone shifts and cosmic radiation exposure in flight attendants. It does not replace a specific individual’s diagnosis or treatment, and actual medical judgment must be made through consultation with a specialist.
※ The images used in this article were generated using generative AI (ChatGPT, OpenAI) as visual references to aid understanding and do not depict real individuals.