Research ArticlePHYSIOLOGY

Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the Moon

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Science Advances  27 Jan 2021:
Vol. 7, no. 5, eabe1358
DOI: 10.1126/sciadv.abe1358
  • Fig. 1 Illuminance and gravimetric cycles that the Moon imposes on Earth.

    (A) Schematic of the movements of Moon and Earth around the Sun (sizes of Sun, Earth, and Moon and distances between them are not to scale). (B) The Sun illuminates the Moon, which causes a 29.53-day cycle (the synodic month) of changing nocturnal illuminance on Earth (full, half, and new moon). The synodic month is ~2.2 days longer than the orbital period of the Moon because Earth has moved in its orbit around the Sun and it takes 2.2 days longer until the Moon is again in its full moon position. The Moon also exerts gravitational effects on Earth that are most evident in the tides. High and low tides occur twice a day with a period of 12.4 hours. Spring tides happen every 14.765 days during full and new moons and neap tides during half-moons. (C) The Moon’s orbit around Earth is tilted with respect to the plane of Earth’s orbit around the Sun (the ecliptic). When the Moon is high in the northern sky, the difference in the amplitude between the two daily tides is maximal in the Northern Hemisphere of Earth. This happens every 27.32 days, and the corresponding cycle is called tropical month. (D) The orbit of the Moon around Earth is elliptic with Earth in one of the focal points of the ellipse. Therefore, the Moon-Earth distance varies cyclically: Every 27.55 days, the Moon is close to Earth (in its perigee) and exerts maximal gravitational forces on Earth, resulting in high tidal amplitudes, while it exerts minimal gravitational forces when it is far from Earth (in its apogee). This cycle is called anomalistic month.

  • Fig. 2 Distribution of periods of the menstrual cycle relative to age.

    Periods were determined by calculating the distances between individual menses for all women for the ages ≤35 years (red) and >35 years (blue). In the box plots, the medians are indicated as horizontal lines and the means are indicated as crosses. A Kruskal-Wallis nonparametric test revealed that the period of the menstrual cycle was significantly shorter at ages >35 years than at younger ages (P < 0.001).

  • Fig. 3 Menstrual cycles in relation to the Moon’s synodic cycles.

    (A) Raster plots of menstrual cycles of six women (birth years in parenthesis) recorded between 1978 and 2015. The raster plots consist of 29.54-day segments of the time series of each individual’s record of menses onsets, which are shown consecutively beneath one another. The segments approximate the 29.53-day period of the synodic month. The plots are repeated three times, as indicated in the plot of subject 6, to facilitate visual inspection of the courses of the menstrual cycles. Onsets of menses are shown as black dots, while times of full moons and new moons are indicated as yellow and blue undulating lines, respectively. Black circles indicate miscarriages; red asterisks indicate births, and triangles indicate gaps in the record. In subjects 1 to 5, the menstrual cycle coupled temporarily to full moons (yellow arrows) or new moons (blue arrows). Light blue shaded areas indicate years of Saros #137, which were characterized by high luminance and high gravitational influence of the Moon during periods when Sun-Earth-Moon syzygies coincided with perigees in which the Moon was exceptionally close to Earth. (B) Periods of menstrual cycles over age. (C) Circular plots of the distribution of menses phases throughout the synodic cycle. The synodic cycle was divided into 30 equal segments each lasting for about 1 day. The number of menses onsets that occurred within a certain segment of the synodic month is plotted radially. Rayleigh tests to check for the presence of a uni- or bimodal distribution with the phases synchronized to the full/new moon (left P values) and for deviations from a uniform frequency distribution of any phase (right P values). Numbers in the lower left corner of each graph indicate the sum of all recorded menses.

  • Fig. 4 Relation between timing of menses and the synodic month in the remaining eight women who kept records for several years before 35 years of age.

    Two of the eight women (subjects 8 and 11) exhibited a rather short cycle length and did not show any synchronization to the synodic lunar month. The other six temporarily followed the full or new moon by visual judgement (see arrows in the raster plots). The Rayleigh tests for uni- or bimodal distribution with central tendency at full or new moon confirmed the latter judgement for three of these women (subjects 9, 13, and 15) and showed a tendency (P < 0.1) for two women (subjects 1 and 14) (left P values in the raster plots). In subject 1, this tendency became significant, when only the first 15 years were included in the analysis. For subject 12, the phase relation between menses onsets and the full or new moon was rather irregular; nevertheless, menses appear to have coupled loosely to the Moon. Labeling as in Fig. 1.

  • Fig. 5 Menstrual cycles temporarily follow the tropical and anomalistic cycles of the Moon.

    (A) Triple raster plot of subject 5 plotted with a period of 27.33 days, which approximates the orbital period of the Moon and the duration of the tropical month (27.32 days). Lunar phases are indicated as colored undulating lines. Time intervals demarcated by stippled lines in the raster plot were assessed by periodogram analyses. The respective periodograms are shown to the left or right of the raster plot. Stippled lines in the periodograms indicate the 5% significance level. The stronger the rhythm and the longer the duration of its presence, the higher the power. Menses temporarily followed the tropical month or the anomalistic cycles or exhibited phases of relative coordination to these cycles (long arrows). After age 46, when the rhythm displayed three instances of relative coordination with the new moon phase of the synodic month (Fig. 3A), sudden changes in period occurred when the cycle coincided with perigees or apogees (open arrows). (B) Circular plots showing the distribution of menses throughout the tropical and anomalistic month (labeling as in Fig. 1). The Rayleigh tests showed that menses distribution significantly deviates from a uniform distribution for both lunar cycles. During the anomalistic month, menses onsets were preferentially directed to apogees. During the tropical month, menses phase distribution was more variable.

  • Fig. 6 Relation of menses to the tropical and anomalistic months in five women who began recording onsets of their menses in their 30s.

    Subject 16 was synchronized with the synodic month during the first year of recording, while the others were never synchronized with the synodic month. Instead, by visual judgment, all five women temporarily followed the anomalistic and tropical months as indicated by colored arrows. Two colored arrows point to the times at which synchronization happened when the two types of cycles coincided. The circular statistics confirmed the alignment of menses onset with the anomalistic and tropical month only in a few cases. Labeling as in Fig. 5.

  • Fig. 7 Menstrual cycle phases are aligned with the phases of full/new moon and perigee/apogee, and this alignment is stronger during winter.

    Left plots: Phase relation between menstrual cycles and the synodic, anomalistic, and tropical months. These plots combine all 4112 menses of the recorded 22 women. While menses onsets significantly align with the time of the full or the new moon and with the time of perigee or apogee, no systematic phase relation of menses to lunar standstills was observed (P values of Rayleigh tests are given at the top left and right corners). The best models of bimodal von Mises distributions are superimposed on the polar plots (red points and red curve). The parameters for these models have been estimated by a maximum likelihood approach (tables S1 to S4). The differences between the best model and a uniform distribution (null model) are expressed as ΔAIC values (right bottom corner in the plots). ΔAIC values >2 indicate peaked distributions, as found for the synodic and anomalistic months but not for the tropical month. For the full moon, the MLE (maximum likelihood model estimations) bootstrapping confidence interval for the central tendency (95% confidence interval between 0.96 and 0.98) was located left of “0,” meaning that most menses onsets occurred ~1 day before full moon (for details, see Material and Methods and the Supplementary Materials). Middle and right plots: Phase relation between menstrual cycles and the synodic, anomalistic, and tropical months separated for spring/summer (April to September) and autumn/winter (October to March). For the synodic month, the alignment with the Moon was highly significant for both times of the year but appeared stronger for autumn/winter. For the anomalistic month, the alignment was only significant for autumn/winter, while it reached no significance at any time for the tropical month.

  • Table 1 Summary menses of recorded subjects.

    Age atNumber of recordedPeriod length in days
    (mean ± SD/median)*
    Percent of time
    synchronized to the
    synodic month (f: full
    moon; n: new moon)
    Percent of time
    synchronized to the
    anomalistic or
    tropical month (a:
    apogee; p: perigee; ma,
    mi: maximum,
    minimum lunar
    standstills)
    Nocturnal
    light
    exposure
    Record
    begin
    Record
    end
    YearsMensesBirths‡≤35 years>35 years≤35 years>35 years≤35 years>35 years
    Subject 1§215332337332.2 ± 4.2/32.028.4 ± 2.1/28.031.8 (f + n)45.0 (f + n)0.014.6
    (a, ma,
    and mi)
    Low
    Subject 2§195031ǁ331429.2 ± 2.1/29.026.8 ± 3.0/26.056.3 (f + n)14.3 (n)7.5 (mi)18.3
    (a + mi)
    Low
    Subject 3245127360030.4 ± 3.7/30.025.5 ± 2.3/25.041.7 (f)13.8 (f + n)3.4 (p)21.4
    (a + p)
    Low
    Subject 4234219259029.1 ± 3.7/29.025.8 ± 2.1/26.065.0 (f)0.028.1
    (a, p, and
    ma)
    33.3
    (a, p, and
    mi)
    Low
    Subject 5255328368227.8 ± 1.8/28.026.7 ± 3.0/27.02.0 (n)25.7 (f + n)62.5
    (a, p, ma,
    and mi)
    39.0
    (a, p, ma,
    and mi)
    Medium
    Subject 6194425ǁ226226.0 ± 4.9/25.024.7 ± 2.4/25.00.00.07.1 (p)0.0High
    Subject 7§255025297230.2 ± 2.9/30.026.4 ± 2.0/26.027.1 (f + n)19.4 (f + n)0.013.8
    (p and ma)
    Low
    Subject 8315120195331.5 ± 6.4/29.025.8 ± 3.0/26.00.00.011.8
    (a and p)
    12.1 (p)High
    Subject 9254217229028.6 ± 2.2/29.027.2 ± 1.4/27.036.0 (f + n)37.5 (f)25.8
    (a and
    ma)
    16.0
    (a and p)
    Low
    Subject 10334714182027.8 ± 4.8/27.026.9 ± 4.7/26.00.00.00.018.2 (p)Medium
    Subject 1129389104126.5 ± 1.6/26.00.017.6
    (a and p)
    High
    Subject 12§12#18685028.5 ± 4.9/28.020.0 (f + n)28.5 (p)Low
    Subject 1318257108030.3 ± 6.2/31.032.3 (f + n)0.0Low
    Subject 143137666030.6 ± 4.9/30.014.3 (n)3.8 (ma)Low
    Subject 15§12#17549032.4 ± 5.6/30.527.3 (f)0.0Low
    Subject 1645549114027.1 ± 4.5/26.05.0 (n)16.7 (p)Low
    Subject 17384911166026.1 ± 3.7/26.00.029.7
    (a and ma)
    Medium
    Subject 18364913180026.5 ± 2.8/26.00.036.3
    (a, p,
    and mi)
    High
    Subject 194551681025.7 ± 2.5/26.00.00.0Low
    Subject 204348570025.4 ± 6.6/24.00.011.3 (mi)Medium
    Subject 21§40465109026.3 ± 5.7/25.00.00.0Medium
    Subject 22365014196026.0 ± 5.0/26.00.019.7 (a, ma,
    and mi)
    Medium
    Summary41121729.4 ± 1.9/28.926.3 ± 0.9/25.923.6%9.5%13.1%17.7%

    *Period was determined by measuring the distances between the menses.

    †Synchronization was determined on the actograms; only intervals at which menses occurred at least three times in a row at full (f) or new (n) moon were judged as synchronized.

    ‡Includes miscarriages; with the exception of subject 21, only the births occurring during the recorded years are included. For subject 21, the births of the two daughters (subjects 12 and 15) are included, although they happened long before she started the recording of her menses.

    §Relations between subjects. Subject 8 is the mother of subjects 1 and 2; subject 21 is the mother of subjects 11 and 14. Actograms of subjects 19 to 21 are not shown in the figures.

    ǁRecording was interrupted for several years.

    ¶Menses oscillated for five to six cycles with a period of 29.5 days and occurred during the waxing moon. These women were judged as not synchronized because the phase of menses onset was not at full or new moon.

    #Recording started at menarche.

    Supplementary Materials

    • Supplementary Materials

      Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the Moon

      C. Helfrich-Förster, S. Monecke, I. Spiousas, T. Hovestadt, O. Mitesser, T. A. Wehr

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      • Figs. S1 to S4
      • Tables S1 to S4

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