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EFFECTS OF SHIFTWORKON SLEEP ANDMENSTRUAL FUNCTIONIN NURSESSusan Labyak a , Susan Lava b , Fred Turekb & Phyllis Zee ca School of Nursing, University ofWashington, Seattle, Washington, USAb Department of Neurobiology &Physiology and Center for CircadianBiology and Medicine, NorthwesternUniversity, Evanston, Illinois, USAc Department of Neurology, NorthwesternUniversity, Chicago, Illinois, USAPublished online: 11 Nov 2010.

To cite this article: Susan Labyak , Susan Lava , Fred Turek & Phyllis Zee(2002) EFFECTS OF SHIFTWORK ON SLEEP AND MENSTRUAL FUNCTIONIN NURSES, Health Care for Women International, 23:6-7, 703-714, DOI:10.1080/07399330290107449

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Health Care for Women International , 23:703–714, 2002Copyright © 2002 Taylor & Francis0739-9332 /02 $12.00 + .00DOI: 10.1080/0739933029010744 9

EFFECTS OF SHIFTWORK ON SLEEP ANDMENSTRUAL FUNCTION IN NURSES

Susan Labyak, PhD, RNSchool of Nursing, University of Washington, Seattle, Washington, USA

Susan Lava, MSDepartment of Neurobiology & Physiology and Center for Circadian Biology and

Medicine, Northwestern University, Evanston, Illinois, USA

Fred Turek, PhDDepartment of Neurobiology & Physiology and Center for Circadian Biology and

Medicine, Northwestern University, Evanston, Illinois, USA

Phyllis Zee, MD, PhDDepartment of Neurology, Northwestern University, Chicago, Illinois, USA

Evidence suggests that shiftwork is associated with menstrual irregularities, repro-ductive disturbances, risk of adverse pregnancy outcome, and sleep disturbances inwomen, yet little has been done to evaluate the effects of shiftwork on menstrualfunction and fertility. The purpose of this study was to evaluate menstrual function,fertility, and pregnancy outcome in nurses working shiftwork, and to examine the rela-tionship of sleep to menstrual function. Sixty-eight nurses < 40 years old completeda survey evaluating sleep, menstrual function, and pregnancy outcome. Fifty-threepercent of the women noted menstrual changes when working shiftwork. Womennoting menstrual changes reported more physiologica l symptoms (p < 0:003), sleptapproximately one hour less when working nights, and reported lengthened time tofall asleep (p < 0:01) when working nights. Findings suggest that sleep disturbancesmay lead to menstrual irregularities, and changes in menstrual function may be amarker of shiftwork intolerance.

Received June 2000; accepted November 2001.We wish to thank Dr. Theresa Horton for her assistance in developing the Nursing Shift-

work Questionnaire. We would also like to acknowledge Drs. Diane-Holditch Davis, SandyFunk, Elizabeth Tournquist, Michael Belyea, and Martha Lentz for their assistance in thepreparation of this manuscript. Funding from the National Institute of Nursing Research,NINR 5F32NRO6954 , and the Center for Circadian Biology and Medicine, Northwestern Uni-versity, Evanston, IL supported this research.

Address correspondence to Susan Labyak, PhD, RN, School of Nursing, University of Wash-ington, Box 357262, Seattle, Washington 98195-7262 , USA. E-mail: slabyak@u.washington.edu

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704 S. Labyak et al.

One of the hallmarks of modern society is the need for a substantial proportionof the people to work when they would normally be asleep and to sleep when theywould normally be awake. These are people engaged in shiftwork (e.g., 4:00 p.m.–12:00 a.m., 7:00 p.m.–7:00 a.m., 12:00 a.m.–8:00 a.m., etc.), working outside thetypical daytime schedule (Monk & Folkard, 1992). Current estimates suggest that15.2 million people in the United States are working non-daytime hours, non-continuous hours, or for extended periods of time (Bureau of Labor Statistics, 1998).Not only is shiftwork a general feature of the work environment for many industriesin modern society, but the number of employees engaged in shiftwork is increasingas the U.S. economy becomes more global, requiring round-the-clock activities.Women constitute over 46% of the U.S. workforce, and approximately 13.7% ofthose women are shiftworkers (Bureau of Labor Statistics, 1998). A number ofwomen employed in the health care industry are working in hospitals, providinground-the-clock nursing care. A growing body of evidence suggests that shiftworkis associated with menstrual irregularities, reproductive disturbances, risk of adversepregnancy, and sleep disturbances in women. However, little research has been doneto evaluate relationships between menstrual/reproductive disturbances and sleep inwomen shiftworkers. We hypothesize that changes in the menstrual function infemale shiftworkers are associated with changes in the 24-hour sleep/wake rhythm.The purpose of this study was to begin to explore the relationship of sleep tomenstrual function in nurses engaged in shiftwork.

INTRODUCTION

Numerous studies have documented that shiftwork is associated with a vari-ety of negative consequences that include an increase in sick leave and visits tohealth clinics, as well as poorer scores on health indices. Speci� c complaints in-clude muscle aches, respiratory infections, and gastrointestinal disorders (generalgastric discomfort, as well as peptic ulcer disease). Shiftwork has also been associ-ated with sleep disturbances, fatigue, feelings of stress, and consumption of drugsand alcohol (Gordon, Cleary, Parker & Czeisler, 1986; Lee, 1992). The physicaland mental maladies associated with shiftwork may be due, in part, to an alterationin the normal phase relationships between various internal rhythms. Evidence sug-gests that women experience greater dif� culty than men in adjusting to shiftwork(Tepas, Duchon, & Gersten, 1993). This may be the result of gender differences indisturbances in the circadian sleep/wake rhythm, including a decline in sleep length,increased fragmentation of sleep, and increased sleepiness and fatigue during theirshift (Escriba, Perez-hoyos & Bolumar, 1992; Lee, 1992). In the Work-Sleep Surveyof 2,988 U.S. shiftworkers (Tepas, Duchon, & Gersten, 1993), female night work-ers age 18–49 reported signi� cantly shorter sleep periods than male night workersof the same age. Some of the gender differences in response to shiftwork may bedue to differences in domestic obligations. For example, family responsibilities inchild rearing and maintaining the home environment may interfere with the sleepschedule and decrease the amount of time available to sleep. Decreasing the adverseeffects of shiftwork is of tremendous importance not only for individual workers,but also for the organizations that require their employees to work during usualsleep hours.

Little research has been done to evaluate the adverse effects of shiftwork onwomen’s health. To date, studies conducted among nurses engaged in shiftwork have

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Effects of Shiftwork 705

focused on sleep and fatigue, mental health, and cardiovascular disease. However,a growing body of evidence suggests that work schedules may have a profoundimpact on reproductive health in women. Reports, thus far, indicate an increasedrisk for spontaneous abortions, preterm births, and low birth weight babies in womenworking irregular hours or rotating shifts for extended periods (Axelsson, Rylander& Molin, 1989; McDonald et al., 1988). Menstrual cycle irregularities and painfulmenstruation also have been reported in women working shiftwork (Messing, Saurel-Cubizolles, Bourgine & Kaminski, 1992; Tasto, Colligan, Skjei & Polly, 1978;Uehata & Sasakawa, 1982). In 1978, at the request of the National Institute ofOccupational Safety and Health, Tasto and colleagues surveyed 1,199 nurses inthe United States to evaluate the health consequences of shiftwork in women.Nurses engaged in shift rotations reported longer menstrual cycles, more symptomsat menstruation, and more menstrual disturbances requiring medical interventionthan women who did not rotate. Since the 1978 NIOSH study, little research hasbeen conducted in the United States to evaluate changes in menstrual/reproductivefunction in female shiftworkers, and the relationship between menstrual changes anddisturbances in the circadian or 24-hour sleep/wake rhythm has not been explored.Some women appear to experience greater dif� culty in adjusting to shiftwork thanothers, reporting a higher incidence of fatigue, sleep disturbances, stress, and gastro-intestinal upset (Lee, 1992). Women displaying this “shiftwork intolerance” may beat greater risk for changes in menstrual function, infertility, and adverse pregnancyoutcomes; however, this has not been evaluated. The goal of this descriptive surveystudy was to explore this relationship.

METHODS

Sample

This study is part of a larger study of nurses working in a large metropolitanhospital in the Midwest. Nurses were recruited via posted � iers, and asked tocomplete a questionnaire. The survey was administered at a central location inthe hospital complex on two different occasions, and at times that were accessibleto nurses working the day, evening, or night shift. We included nurses less than40 years of age who met the following criteria: 1) reported regular menstrual periodsduring most of their life; 2) did not report diagnosis of menstrual dysfunction (e.g.,history of endometriosis, amenorrhea, etc.); 3) had not gone through menopauseand/or were not experiencing peri-menopausal symptoms; and 4) had not undergoneuterine surgery including tubal ligation, hysterectomy and/or ovariectomy. Sixty-eight nurses (66 RNs and 2 LPNs) ranging in age from 22–39 years with a meanage of 29.97 ± .6 years were included in the menstrual analysis. Participants hadworked in their current position for at least six months. The average length oftime that respondents had worked shiftwork was 3.3 years (± 5 months), with arange of 6 months to 13 years. If nurses were not currently engaged in shiftwork,they were asked to re� ect on their previous experience of being a “shiftworker” andrespond to questions accordingly. In this study, shiftwork was de� ned as working theevening shift (e.g., 3:30 p.m.–12:00 a.m.) or working the night shift (e.g., 12:00 a.m.–8:00 a.m.), noting that some women worked 12- or 16-hr shifts. Racial distribution ofthe group consisted of 85% Caucasian, 8% African American, 4% Asian or Paci� cIslander, and 3% Hispanic.

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706 S. Labyak et al.

Measures

Nurses were asked to complete the Nursing Shiftwork Questionnaire, a three-partsurvey consisting of: 1) a reproductive history; 2) a sleep history; and 3) demographicinformation. The survey was developed for the study since no instrument was avail-able to measure reproductive and sleep/wake characteristics in female shiftworkers.To elicit information on the in� uence of shiftwork on reproductive health and sleep,we modi� ed a reproductive-history questionnaire and a general sleep questionnaireused by a local hospital. In the reproductive portion of the questionnaire, nurses wereasked about general reproductive health and disease, speci� cally the regularity andduration of the menstrual cycle, use of hormones and contraceptives, the intensityof menstrual � ow, and dif� culties with fertility and pregnancy. Respondents wereasked to report changes in menstrual cycle regularity, menstrual � ow, pain or othersymptoms associated with menstruation, and the duration of the menstrual cyclewhen working the day shift, evening shift, or night shift. In all surveys, nurses wereasked to respond retrospectively if they were not currently working shiftwork.

In the general sleep history section, subjects were asked to report on variousaspects of sleep, including satisfaction with sleep patterns, length of time to fallasleep (sleep latency), the average number of awakenings from sleep, and averagesleep length when they worked the day shift, the evening shift, or the night shift. Togain a better understanding of the types of stressors in their lives, nurses were askedto respond to questions about lifestyle, family responsibilities, and work schedule,including the length of time they had worked shiftwork.

Experts in the � eld of biological rhythms and survey research reviewed the survey,and the items in the questionnaire were judged to have content validity as a measureof reproductive health and sleep. The reliability of the questionnaire was examined byasking 3 nurses to complete the survey on two occasions one week apart. Reliabilitywas computed as the proportion of agreement between the individual’s responses onthe items at Time 1 and Time 2. There was an average of 90.3% agreement acrossthe time periods.

Data Analysis

Mixed general linear model analyses (random regression models) were used toexamine: 1) individual differences in mean sleep latency, number of awakeningsfrom sleep, and sleep length in women when they worked the day shift, the eveningshift, or the night shift; and 2) group differences in mean sleep latency, number ofawakenings from sleep, and sleep length in women reporting changes in menstrualfunction when working shiftwork, and in women reporting no change in menstrualfunction with shiftwork. We used the mixed model rather than MANOVA becausemore than 5% of the nurses had cells without data (e.g., some respondents reportedsleep latency, number of awakenings, and/or sleep length on two of the three shifts,but did not report on a shift that they had not worked). In addition, use of themixed model allowed us to avoid bias in estimation and hypothesis testing. Themixed model analysis uses an iterative process to � t the best estimates for � xedeffects (group regression line of menstrual changes, shift, and interaction of men-strual changes and shift) and for random components (deviation of each individualfrom the group regression). Thus, each individual has her own regression line—a feature that allows the mixed general linear model to accommodate missing data“completely at random” without requiring that missing data be estimated. In mixed

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Effects of Shiftwork 707

model analysis, more than one source of random variability is present (variabilityamong individual differences in intercepts and slope due to error); therefore, nooverall statistic can be calculated, and only the signi� cance of individual predictorsis reported (Holditch-Davis, Edwards & Helms, 1998). Chi Square tests were doneto examine group differences on demographic variables and reported physiologicalsymptoms of shiftwork intolerance.

RESULTS

Menstrual Function and Shiftwork

Thirty-six (53%) of the nurses in the study reported changes in their menstrualfunction when working shiftwork, while 32 women (47%) reported no changein menstrual function with shiftwork. Comparisons between these 2 groups anda summary of fertility, miscarriages, and pregnancy outcome data can be foundin Table 1. The 3 nurses who reported infertility were working shiftwork duringthe time they were attempting to conceive. Furthermore, 5 of the 9 women whoreported having a miscarriage noted that they were working shiftwork at the timeof the incident, and 3 of these women reported changes in menstrual function withshiftwork. We do not know how long these women worked shiftwork prior to theinfertility or miscarriage. A majority (78%) of the women in both groups reportedtaking oral contraceptives.

Table 2 provides a summary of changes reported by women when working shift-work. The most commonly cited change in menstrual function related to menstrual� ow, with some women reporting an increase in blood � ow during menstruation andothers reporting a decline. As can be seen in Table 2, responses were variable andreported changes in menstrual function were not always in a predicted direction.

Sleep Characteristics

Shift-related Changes in Sleep for the Total SampleRespondents were asked to report their average sleep latency, number of awak-

enings from sleep, and sleep length when they worked the day shift, evening shift,and night shift (see Table 3). Women reported signi� cantly more awakenings fromsleep when working the night shift than when working day or evening shifts, andthey reported signi� cantly longer sleep episodes when working the evening shiftthan when working either day or night shifts. No signi� cant shift-related differenceswere noted in sleep latency. Women noting menstrual changes when working shift-work reported signi� cantly longer sleep latencies when working the night shift thanwomen who reported no menstrual changes. No signi� cant group differences werenoted in number of awakenings from sleep or sleep length.

Sleep and Menstrual FunctionIn the survey, women were given a checklist of physiological symptoms com-

monly reported by individuals having dif� culty tolerating shiftwork and were askedto identify symptoms that they experienced when working shiftwork. A summaryof those responses can be found in Table 4. Compared with nurses who did notexperience changes in menstrual function, women noting menstrual changes with

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Tab

le1.

Sam

ple

dem

ogra

phic

data

Age

Hor

mon

eC

hild

ren

Shif

tSh

ift

dura

tion

Rep

rodu

ctiv

eM

ean

±SE

MM

arit

alst

atus

use

12yr

sol

dw

orke

d(h

rs)

dif�

cult

ies

Gro

up(r

ange

)n

(%)

n(%

)an

(%)

n(%

)n

(%)b

n(%

)

Men

stru

alch

ange

sw

ithsh

iftw

ork

(nD

36)

30.3

±0.

8yr

sM

arri

ed:

11(3

1)28

(78)

7(1

9)D

ays:

5(1

4)8–

10hr

s:14

(39)

Infe

rtil

ity:

2(6

)(2

2–yr

s)S

ingl

e:22

(61)

Eve

ning

s:3

(8)

12hr

s:19

(53)

Mis

carr

iage

:4

(11)

Div

orce

d/w

idow

ed:

3(8

)N

ight

s:6

(17)

16hr

s:3

(8)

Pro

blem

preg

:1

(3)

Rot

atin

g:22

(61)

No

men

stru

alch

ange

sw

ithsh

iftw

ork

(nD

32)

29.6

±0.

9yr

sM

arri

ed:

14(4

4)25

(78)

8(2

5)D

ays:

8(2

5)8–

10hr

s:7

(22)

Infe

rtil

ity:

1(3

)(2

3–39

yrs)

Sin

gle:

16(5

0)E

veni

ngs:

1(3

)12

hrs:

22(6

9)M

isca

rria

ge:

5(1

6)D

ivor

ced/

wid

owed

:2

(6)

Nig

hts:

4(1

3)16

hr:

3(9

)P

robl

empr

eg:

0R

otat

ing:

19(5

9)

aC

urre

ntly

taki

ngor

had

prev

ious

lyta

ken

repr

oduc

tive

horm

ones

for

cont

race

ptio

n.bA

typi

cal

wor

kwee

kfo

rw

omen

who

wor

ked

16-h

our

shif

tsco

nsis

ted

oftw

o16

-hou

rsh

ifts

and

one

8-ho

ursh

ift

per

wee

k.

708

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3

Effects of Shiftwork 709

Table 2. Reported changes in menstrual function

Variable % Nurses reporting change Total %

Menstrual cycle length 3 Shortens, 9 lengthens, 10 varied 22Menstrual � ow 9 Increases, 10 decreases, 16 varied 35Menstrual pain 18 Increase in pain 18Duration of bleeding 9 Shortens, 7 lengthens, 4 varied 20

shiftwork (1) reported signi� cantly more sleep disturbances (Â2 [1; N D 68] D10:35, p < :01), dif� culty concentrating at work (Â2 [1; N D 68] D 4:39; p < :05),incidences of nervous stomach (Â2 [1; N D 68] D 4:09, p < :05) and generalmalaise (Â2 [1; N D 68] D 10:24, p < :01); (2) slept an average of 1 hour less onnights compared with sleep length when working days (nights: 5.9 ± .3 hrs; days:6.9 ± .2 hrs), and (3) reported signi� cantly longer sleep latencies when workingnights. The average sleep length for women who did not experience changes inmenstrual function was 6.6 ± .4 hrs on nights and 6.5 ± .1 hrs on days.

The possibility remains that nurses who work rotating shifts may have a differentoutcome than nurses working permanent nights. Using Chi Square analysis, welooked at differences between nurses who rotated and nurses who worked the nightshift. The only signi� cant � nding was that night nurses were older than nurses whoworked rotating schedules (night nurses 32.2 ± 1.8 years, rotating nurses 28.3 ±.7 years; Â2 [1; N D 51] D 32:29, p < :01). There were no signi� cant differencesbetween the two groups on either menstrual or sleep variables.

DISCUSSION

Shiftwork is a general feature of the work environment for the health care industry,and a high percentage of nurses are engaged in shiftwork. The � ndings in this studysuggest that changes in menstrual cycle function and a higher incidence of painfulmenstruation occur among many female nurses working nights and rotating shifts.Fifty-three percent of the respondents in this sample reported changes in menstrualcycle function when working shiftwork. Nurses reporting changes in menstrualfunction noted signi� cantly more sleep disturbances and dif� culty concentrating atwork, a higher incidence of other physiological symptoms of shiftwork intolerance,and signi� cantly longer sleep latencies when working shiftwork than nurses whodid not report menstrual changes. While it is not clear if menstrual problems causedproblems with sleep, or vice versa, the clear relationship between them indicatesthat the disturbance of sleep is related to menstrual irregularities.

The literature suggests an increased risk of spontaneous abortion, preterm births,and low birth weight babies among women working irregular hours, rotating shifts,or for extended periods (Axelsson, Rylander & Molin, 1989; McDonald et al., 1988).In the United States, an estimated 10%–15% of all couples report infertility of 1 yearor longer, 15%–20% of clinically recognized pregnancies end in spontaneous abor-tion, 7% of infants born in the United States are born underweight or prematurely,and 2%–3% of infants born in the United States are born with a major birth defect(National Institute of Occupational Safety and Health, 1999). Of the 68 participants

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Tabl

e3.

Eff

ects

ofm

enst

rual

chan

gean

dsh

ift

onsl

eep

Shif

tF

test

s

Men

stru

alIn

tera

ctio

nM

enst

rual

Day

Eve

ning

Nig

htch

ange

Shif

t(m

enst

rual

chg

shif

t)V

aria

ble

chan

ge(X

+SE

M)

(X+

SEM

)(X

+SE

M)

F(d

f)F

(df)

F(d

f)

Slee

pla

tenc

yY

es22

.25

(±3.

64)

40.3

8(±

6.28

)45

.69

(±8.

73)

(min

utes

)N

o29

.74

(±4.

43)

24.0

7(±

3.71

)24

.15

(±5.

16)

3.05

(1,6

5)2.

06(2

,65)

5.52

(2,6

5)¤

#A

wak

enin

gsY

es1.

15(±

0.22

)1.

29(±

0.20

)2.

86(±

0.40

)N

o1.

27(±

0.22

)1.

14(±

0.23

)2.

02(±

0.28

)1.

09(1

,66)

10.7

9(2

,66)

¤¤1.

27(2

,66)

Slee

ple

ngth

Yes

6.91

(±0.

20)

7.24

(±0.

34)

5.92

(±0.

30)

(hou

rs)

No

6.48

(±0.

14)

7.88

(±0.

34)

6.60

(±0.

40)

0.52

(1,6

5)11

.83

(2,6

5)¤¤

¤2.

14(2

,65)

Pos

tho

cco

ntra

sts:

¤ p<

:01;

sign

i�ca

ntly

long

ersl

eep

late

ncie

sw

hen

wor

king

the

nigh

tsh

ift.

¤¤p

<:0

01;

sign

i�ca

ntly

mor

eaw

aken

ings

whe

nw

orki

ngni

ght

shif

tth

anw

hen

wor

king

day

orev

enin

gsh

ift.

¤¤¤ p

<:0

01;

sign

i�ca

ntly

long

ersl

eep

leng

thw

hen

wor

king

even

ing

shif

tth

anw

hen

wor

king

the

day

orni

ght

shif

t.

710

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3

Effects of Shiftwork 711

Table 4. Physiological symptoms when working shiftwork

Women reporting Women reporting nomenstrual changes menstrual changes

with shiftwork with shiftworkPhysiological symptom n (%) n (%) p value

Sleep disturbances 31 (86) 16 (50) p < 0:01Fatigue & drowsiness at work 25 (69) 19 (59) NSGeneral malaise 19 (53) 5 (16) p < 0:01Muscle aches 12 (33) 8 (25) NSNervous stomach 12 (33) 4 (13) p < 0:05Nausea 12 (33) 8 (25) NSDif� culty concentrating at work 9 (25) 2 (6) p < 0:05Constipation 7 (19) 5 (16) NSRespiratory infections 5 (14) 3 (9) NSDiarrhea 4 (11) 1 (3) NSAbdominal cramps 2 (6) 2 (6) NSOther 8 (22) 8 (25) NSNo symptoms checked 1 (3) 6 (19) p < 0:05

in this study, 4.4% of the women reported experiencing infertility for one year orlonger, 13.2% reported a miscarriage, and 1.5% reported a problem pregnancy. Thesmall sample size in this study did not enable us to evaluate whether or not these rareevents occurred more often in female shiftworkers. However, women in the samplewho reported changes in menstrual function with shiftwork did not appear to be atgreater risk for infertility or the occurrence of a problem pregnancy.

Research on the impact of occupational variables on human reproductive functionhas largely focused on pregnancy outcome rather than menstrual cycle function andfertility. The rate and quality of ovarian follicular growth and development deter-mine the length of the menstrual cycle. In the general population, approximately 20%of women in their reproductive years report irregular menstrual cyclicity (Speroff,Glass & Kase, 1994). Fifty-three percent of the women in this sample reportedchanges in menstrual cycle function when working shiftwork, including changes inthe length of the menstrual cycle (22%), changes in menstrual � ow (35%), increasein menstrual pain or dysmenorrhea (18%), and changes in the duration of men-strual bleeding (26%). These � ndings agree with the � ndings of earlier studies inwhich investigators reported an increase in menstrual cycle irregularities and painfulmenstruation among female shiftworkers (Messing, Saurel-Cubizolles, Bourgine &Kaminski, 1992; Tasto, Colligan, Skjei & Polly, 1978; Uehata & Sasakawa, 1982).Messing and colleagues (1992) examined menstrual cycle function in 726 femalesworking in industry, and noted that long menstrual cycles were associated with workschedule variability. Long menstrual cycles could result from a delay in the follicularphase, suggesting alterations in follicular development. In a survey of 2,264 Japanesewomen, complaints of irregular menstrual cycles and menstrual pains were signif-icantly greater among night shift workers than day workers (Uehata & Sasakawa,1982). Menstrual cycle irregularities and exacerbated dysmenorrhea also have beenwidely reported among airline stewardesses working irregular schedules and repeat-edly experiencing time-zone changes (Preston, Bateman, Short & Wilkinson, 1973).

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712 S. Labyak et al.

Though shiftwork has been associated with reproductive dysfunction in women,the neuroendocrine mechanisms responsible for this dysfunction are poorly under-stood. Most of the studies that have reported menstrual cycle disturbances weredescriptive reports. Preston, Bateman, Short, and Wilkinson (1973) attempted to ex-plain the menstrual cycle irregularities that they observed in airline stewardesses byhypothesizing that environmental stresses, such as repeated time-zone changes, couldpostpone or suppress ovulation in women. If the stressor was introduced during thepre-ovulatory phase, then ovulation was either inhibited or delayed, and menstrua-tion was postponed. If the stressor was presented during the post-ovulatory phase,then menstruation occurred at its predicted time; however, persistent exposure to thestressor could delay the timing of ovulation in the next cycle. No attempt, however,was made to determine what endocrine abnormalities were associated with speci� calterations in menstrual cycle function. Asso (1983) suggests that the menstrual ir-regularities observed in airline stewardesses may partially be attributed to the effectsof high altitude. High altitudes have been associated with disturbances in menstrualfunction, and may have an adverse effect on fertility.

Reproductive disturbances associated with shiftwork may be due, in part, to a dis-ruption of normal circadian and sleep-regulated 24-hour rhythms. Sleep disturbancesamong women engaged in shiftwork are well documented (Escriba, Perez-hoyos &Bolumar, 1992; Lee, 1992). In the present study, nurses reporting changes in men-strual function noted signi� cantly more sleep disturbances coupled with a higherincidence of physiological symptoms of shiftwork intolerance. Circadian rhythmdisturbances of growth hormone, prolactin, cortisol, thyroid stimulating hormone(TSH), melatonin, luteinizing hormone (LH), and catecholamines have been iden-ti� ed in night shiftworkers. Sleep is normally associated with a signi� cant slowingof LH pulsatility as well as a signi� cant increase in the amplitude of the LH pulsein women during the early follicular phase of the menstrual cycle. The evidencesuggests that partially sleep-deprived women demonstrate an increase in serum LHand a decline in serum prolactin (PRL) levels (Baumgartner et al., 1993). No one,however, has examined this in female shiftworkers, who often report chronic sleepdisturbances. Shiftwork is also associated with an increase in stress, and stress-related dysfunction of the hypothalamo-pituitary-ovaria n axis is well documented.The combination of physiological complaints related to circadian disruption and thestrain of continuing to maintain social and family roles and responsibilities con-tributes to the stress of female shiftworkers. Though we did not measure stress inthis study or explore changes in mood, these are important variables that need to beconsidered in future studies.

The � ndings of this descriptive study suggest a relationship between sleep dis-turbances and changes in menstrual function in female shiftworkers. Women whoreported changes in menstrual function when working shiftwork noted signi� cantlymore sleep disturbances, feelings of general malaise, and dif� culty concentratingat work compared with nurses who reported no menstrual changes. Chronic sleepdeprivation leads to decrements in alertness and performance, certainly major is-sues for health care workers often working in critical areas. Recent evidence alsosuggests that sleep plays a role in normal regulation of the immune system. Sleeploss may also result in compromised immune function and increased susceptibilityto infection (Benca, 1995). In Lee’s (1991) survey of nearly 800 nurses working avariety of shifts, nurses who worked night or rotating shifts reported more healthproblems and menstrual irregularities than those working other shifts.

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Effects of Shiftwork 713

A limitation of this study is that some respondents were not working shiftwork atthe time they completed the questionnaire, and were, therefore, recalling experiencesfrom the past. The survey did not prompt participants to indicate how long it hadbeen since they had worked shiftwork. Concerns regarding the accuracy of participantrecall need to be considered in future studies, as well as the length of time that a nurseworked a particular shift. A larger sample is needed to further establish reliabilityand validity of the questionnaire. The goal of future research will be to utilize a morerigorous, prospective study design involving a larger sample of nurses, focusing ona homogenous sample of nurses working shifts of a particular length or making thetransition to changing shifts. Such studies could incorporate the use of a daily diary,prompting women to report on the menstrual, sleep, and reproductive variables ofinterest. Using this protocol, a longitudinal evaluation of sleep and menstrual functioncould be conducted in women currently working shiftwork and in a control groupof women who do not work shiftwork.

CONCLUSION

Shiftwork rotation often results in temporal disturbances in circadian rhythmssuch as the sleep/wake and body temperature rhythms, as well as other internal bio-logical rhythms. Biological rhythms play an important role in the regulation of manyphysiological systems, including the reproductive system. In particular, the 24-hourclock system plays a role in regulating the production, release, synthesis and action ofreproductive hormones, and in the timing of critical events in the ovarian cycle. Therehas been little attempt to explore the physiological and endocrine mechanisms thatunderlie the relationship between disturbances in the temporal organization of bio-logical rhythms and reproductive dysfunction in shiftworkers. Future research effortsneed to include a more in-depth evaluation of reproductive disturbances in femaleshiftworkers and an exploration of the potential underlying neuroendocrine changesthat accompany these disturbances. Incorporating physiologic variables, such as thetiming of ovulation as a marker of menstrual function and salivary cortisol as amarker of stress, will be important in terms of gaining a better understanding ofmenstrual disturbances that accompany shiftwork. In addition, it will be importantto evaluate other intervening variables such as the use of pharmacological agentsand pre-existing health conditions that may interfere with sleep. Our � ndings sug-gest that disturbances in sleep may lead to menstrual irregularities, and some womenmay be more vulnerable than others to the negative effects of shiftwork. Changesin menstrual function may be a marker of this shiftwork intolerance. In terms ofpolicy implications, the � ndings suggest the need for employers and workers’ occu-pational groups to work toward the development of employee policies that minimizefrequently changing work schedules. A crucial � rst step would be for employers toschedule shifts in a way that minimizes the need for shift rotation, particularly forindividuals who display shiftwork intolerance. Implementing schedules that allownurses to obtain suf� cient rest and recuperation between varying work schedulesshould be a top priority—one that will enhance employee health and productivity.

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Speroff, L., Glass, R. H., & Kase, N. G. (1994). Clinical gynecologic endocrinology and infertility(5th ed.). Baltimore: Lippincott Williams & Wilkins.

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