AGE, WORKING CONDITIONS, AND SLEEP DISORDERS
Age, Working Conditions, and Sleep Disorders: a
Longitudinal Analysis in the French Cohort E.S.T.E.V.
Céline Ribet (MPH)1 and Francis Derriennic (PhD)2
1INSERM
U258, Hôpital Paul Brousse; 2INSERM U170, Hôpital Paul Brousse
Study objectives: To investigate the effects of occupational factors on both the incidence and the disappearance of sleep disorders after a five–year follow–up period.
Design: A prospective longitudinal investigation E.S.T.E.V. carried out in 1990 and 1995.
Setting: Seven regions of France.
Subjects: A random sample of employed men and women born in 1938, 1943, 1948, and 1953. In 1990, 21,378 subjects were interviewed (87% of those contacted), and 88% were interviewed again in 1995.
Measures: Sleep disorders (SD), objectifiable and psychosocial working conditions.
Results: Prevalence of SD increased with age and were more frequent among women than men in every age group. Incidence of
SD varied little with age, but their disappearance decreased with age. After adjustment for age and sex, SD in 1995 were found to
be associated both with objectifiable working conditions and with psychosocial aspects of the way work is experienced. Among
objectifiable occupational risk factors, shift work, work week often longer than 48 hours, and exposure to vibrations appeared to be
the principal risk factors for SD. Among psychosocial occupational factors, finding it difficult or irksome to have to hurry appeared
to be the principal risk factor.
Conclusions: Taking into account the adjustments for health criteria, sociodemographic characteristics, and leisure activities, these
results suggest useful courses of action for prevention which, it seems to us, must not be only limited to objectifiable working conditions. Issues about work organization, while clearly difficult to resolve, must also be taken into account.
Key words: Sleep disorders; age; sex; job characteristics; psychosocial factors
INTRODUCTION
higher consumption of tobacco, coffee or tea, laxatives,
sleeping pills, analgesics, and cough–relieving medication,9 as well as alcohol, which 28% of insomniacs use to
help them sleep.10 All of these reasons help explain why
SD appear to be closely related to automobile accidents and
workplace accidents, injuries, and errors. According to the
literature, the percentage of traffic accidents directly due to
problems of somnolence or fatigue ranges between 2.5%
and 13%.6,11–13 Similarly, many industrial accidents,14–16
notably in nuclear plants,17,18 are associated with wide
weekly variations in work hours which induce disruptions
in the quantity and quality of sleep.
SD have been studied to uncover their causes in various
sectors of occupational activity and to itemize their monetary cost. These disorders lead to both direct and indirect
costs, due to absenteeism, diminished productivity, reimbursement for medications and for treatment of the ailments that can result from SD.10–12
Assessment of SD and of their prevalence and research
into the risk factors that might be associated with them are
thus an important public health objective and are indispensable for prevention. Nonetheless, this assessment and
this research can be difficult, especially because prevalence
SLEEP DISORDERS (SD) INCREASE with age and may
be considered as possible markers of aging.1 The processes
involved are complex and multidimensional (biological,
psychological, social). The alternation between wakefulness and sleep is part of the circadian rhythms necessary for
functional equilibrium and may suffer long–term
after–effects in case of dysfunction.2–6 In any case, sleep
involves several phases of a structure that changes over the
course of a lifetime; these phases are thus an example of
circadian rhythm change as a function of age.3,6,7
Studies have shown an association between circadian
rhythm disorders and the disruption of some human performances, including diminution of attention, motivation,
ability to concentrate, and a slowing down of the process of
perception–motricity.8 Moreover, SD are associated with a
Accepted for publication January 1999
Correspondence : Céline Ribet, Inserm U258, Hôpital Paul Brousse, 16 Avenue
Paul Vaillant-Couturier, F94807 Villejuif Cedex, Tel: 01 45 59 51 01, Fax: 01 45
59 51 52, E-mail: [email protected]
SLEEP, Vol. 22, No. 4, 1999
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
can vary between 9% and 31%, depending on the definition
of SD.19
Some socio–demographic factors and some objectifiable
working conditions are generally accepted to be risk factors
for SD. In particular, the primary risk factor, according to
nearly all epidemiologic studies, is age. Despite the modification of circadian rhythms as a function of age, the duration and quality of sleep decrease with age.20–30 Many studies have also found that sex is a possible risk factor for SD,
as they are more prevalent among women than
men.19,21–23,25,27,31,32
Looking at occupational factors requires simultaneous
consideration of objectifiable work or job characteristics
and factors related to the way the work situation is experienced (that is, psychosocial factors). The first indicators
correspond to indicators that can be assessed by an independent observer and may be quantifiable. Of these, shift
work (rotating schedules, early morning, late evening,
night) has been examined in detail, and it has been repeatedly found to be associated with SD in cross–sectional
studies.6,27,29,30,33–43 To this notion of dividing up the time
devoted to occupational activity is added the rhythm of
work by time unit. Thus, work under time constraints
(assembly line or piece work), especially among women,
appears to be a risk factor for SD.25,29,30 The mechanisms
by which this occurs are thus more subtle (increased
fatigue, nervous tension, etc...).
Occupational exposure to loud noise is a factor associated with poor quality sleep,30,44 although this relation is not
always present.25 Some studies indicate that work overload,
either physical or mental, may play a role.25,45 On the other
hand, no association has been found between SD and
seniority.42,43
Psychosocial factors, on the other hand, imply a subjective judgment by workers about their work and are assessed
in interviews or with self–questionnaires. Specific psychosocial factors that have been found to be related to SD
are a negative perception of human relations in the workplace, whether with colleagues, supervisors, or superiors.25,32,45 This perception may result from a negative or
stressful work environment. Estryn–Behar et al.29 have
shown that excess SD among hospital caregivers were
associated with insufficient in–house training and insufficient coordination of services. Other factors have been
mentioned as possible risk factors for SD, including the
degree to which the employee chose his or her occupation,31 his or her interest in the work,25 how diversified it
was,25,29,31 and over–involvement in work.46 Nonetheless,
Tachibana et al.46 found no significant relation between SD
and interest in the work, human relations at the workplace,
and social support at work.
Too few studies have taken into account simultaneously
and in a detailed fashion both psychosocial and objectifiable work factors, and as far as we know there is no longiSLEEP, Vol. 22, No. 4, 1999
tudinal study involving these factors together. Therefore,
many questions remain unanswered about the causal role of
these factors. The first of the uncertainties is age, because
the effects linked to a particular generation and those specific to age itself are not always clearly distinguished. For
this reason, it seemed useful to use the data collected as part
of the longitudinal French ESTEV study (Health Work and
Aging Investigation), among a large representative sample
of employees of various occupations and both sexes, to
uncover the occupational factors related to the evolution of
SD, in terms of both incidence and cure, after a prospective
five–year follow–up between 1990 and 1995.
Population and methods
The goal of ESTEV, a prospective longitudinal epidemiologic investigation on which this study was based, is to
identify occupational factors that might modify the evolution over time of some health characteristics. The sample
population in 1990 was randomly selected from the complete lists of employees treated by occupational physicians
in the private sector. The occupational physicians participated on a voluntary basis and came from seven regions of
France that offer a contrast from a socio–economic point of
view: Bretagne, Dauphiné, Ile de France, Nord, Pays de
Loire, Val de Loire, Ile de la Réunion.
For each physician, the sample selection was stratified
by sex and the four years of birth considered (1938, 1943,
1948, and 1953), by varying the sampling rate per stratum
according to the distribution of each stratum among all
French employees. These weights were deduced from the
national employment statistics.
A total of 21,378 employees participated in the 1990 survey. The rate of participation was 88% and did not vary by
more than 1% by age, sex, or region. Of the 21,378 responders in 1990, 18,695 were interviewed again in 1995 (i.e. a
follow–up rate of 87%). Of these subjects, 16,833 were still
at work in 1995 (91% of the follow–up sample).
The survey took place, both in 1990 and 1995, during
the annual occupational medical examination. To homogenize the data collection procedure, all participants, including those no longer working in 1995, had been convoked
and interviewed again by their occupational physician.
Participants completed three questionnaires with closed
questions, identical on both dates.
The first was a two–part occupational questionnaire. The
first part, filled in by the employee alone or with the physician's help, concerned past or present (at the time of the
survey) working conditions. The second part, filled in solely by the employee, concerned the employee's assessment
of the good and bad points of the job.
The second questionnaire was also completed solely by
the employee; it concerned his or her principal social characteristics, living conditions, and perceived health.
Finally, the physician completed a medical questionnaire
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
that listed past and current disorders, age at first appearance
of each disease and any treatment prescribed, clinical and
anthropometric data, and results of additional examinations.
– 3: at least two constraints reported in 1990.
A subject was considered as being exposed to a given constraint if he stated that he was exposed to that constraint for
more than 50 working days in the year ;
3) The variables of occupational exposure, including:
awkward postures, carrying heavy weights, exertion
required to operate tools or machines, rapid or frequent
walking required, vibrations, loud noise. There were three
possible codes for each of these : exposed at work at the
time of the interview, not exposed now but exposed in the
past, never exposed.
Exposure to bad weather, cold, or heat, on the one hand,
and exposure to dust or smoke, microbes or other infectious
agents, or chemical products, on the other hand, were also
considered. For these two types of exposure, the coding is
identical to that used for shift work.
The psychosocial factors included two groups. The first
group involved a global assessment of the workplace with
respect to:
– the means available to carry out high quality work
(material, information, time),
– the possibility of choosing oneself the way in which
the work will be carried out,
– the diversification of work,
– the learning of new things at work.
The second group of psychosocial factors is represented
by a group of variables that assess the difficulties of the
working conditions or requirements of the occupation:
– to have to hurry, to have to do several things at the
same time, to be interrupted often while working,
– to notice very tiny details or read badly written texts,
– to use very precise movements,
– to remain poised in dangerous situations, to work
overtime, to put up with the demands of the public,
and finally
– to be required to keep one's eyes constantly on one's
work or not be able to take interruptions.
Each of these variables could be coded three ways:
unconcerned or concerned, the latter subdivided into the
constraint was stated to be difficult or not.
To this group of factors we added the employee's status
in 1995, using the dichotomous variable "still at work in
1995: yes or no." Among the subjects followed–up, 9.3%
were no longer employed: 42% were unemployed, 23% not
working for reasons of health, and 35% retired. Most of the
latter group came from the group of subjects born in 1938.
Sleeping problems
In 1990 and 1995, SD were assessed according to the
five items of the "sleep" dimension in the French version,
developed by Bucquet,47 of the Nottingham Health Profile
(NPH). This indicator allows to define six dimensions of
perceived health, four of which are predominantly physical
and two predominantly psychological. The items composing each dimension are weighted according to the subjective severity of the problems and symptoms reported by the
subjects in the studies carried out to develop that indicator.
The sleep dimension includes the following five items:
1) I take tablets to help me sleep;
2) I lie awake for most of the night;
3) I sleep badly at night;
4) It takes me a long time to get to sleep;
5) I'm waking up in the early hours of the morning.
Subjects were considered as having SD if they mentioned at least two of the five possible items, with at least
one of the first three (which had a weight greater than 20).
This means that the notion of SD has been "tightened" both
quantitatively (at least two items) and qualitatively (at least
one "severe" item).
Occupational variables in 1990
The objectifiable characteristics of work were divided
into three homogeneous subgroups of variables, comprising:
1) Variables of the individual job history, including
socioeconomic category, age at first employment, number
of years of unemployment since the start of working life,
work in the public sector, whether the occupation was a
personal choice;
2) Time–related variables, associated with work schedules and rhythms: work week often (more than 20
weeks/year) longer than 48 hours, rotating shift work,
repetitive work under time constraints, payment on a piecework basis, and work hours imposing constraints about
specific times for sleep (going to bed after midnight, getting up before 5 o'clock in the morning, not sleeping at
night). This last variable has been called shift work in the
multifactorial analysis and coded as follows:
– 0: no constraint reported in 1990 or in past work
history,
– 1: no constraint in 1990, but at least one in past
work history,
– 2: one constraint reported in 1990, and
SLEEP, Vol. 22, No. 4, 1999
Non–occupational variables
Leaving aside age and sex, we considered four subgroups of variables:
1) sociodemographic variables including country of
birth, marital status, number of children, highest
educational level achieved;
2) variables characterizing leisure activities, including
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
sports, gardening, odd jobs, knitting and/or sewing,
social activities, cultural and/or artistic activities and
watching television;
3) health variables observed during clinical examination and including musculo–skeletal pains which subjects had suffered for more than six months at the time
of the survey, the number of past and present (at the
time of the survey) disorders. These disorders were
included in the analysis coded as one of the 17 main
chapters of the 9th International Classification of
Diseases;
SLEEP, Vol. 22, No. 4, 1999
4) perceived health variables affecting energy, social
isolation, physical mobility. These variables, like the
"sleep" dimension, come from the NHP scale.
Statistical analysis
The same analysis was planned to examine the role of
various factors on the incidence of SD (among subjects not
suffering from SD in 1990) and on the disappearance of SD
(among subjects who stated they had SD in 1990) after the
five–year follow–up.
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
Bivariate analysis
RESULTS
The bivariate analyses concerned the association, adjusted for age and sex, between SD frequency and each of the
subgroups of variables described above.
These associations were tested with the
Mantel–Haenszel chi–square test, with 5% as level of significance.
Description of the sample population by age and sex
In 1990, the distribution by age was balanced for both
sexes, reflecting both the composition of the sample population and an equal participation in the survey by age for
each sex (Table 1).
In 1995, the distribution by age remained balanced for
the first three cohorts, for both sexes. Also for both sexes,
however, the proportion of subjects born in 1938 and follow–up in 1995 was lower. The number of subjects lost to
follow–up was thus proportionally higher for the oldest
generation. For that generation, there were also more subjects no longer working in 1995 among those seen twice.
Multivariate analysis
Multivariate analysis was performed for both components of this study, the incidence and disappearance of SD,
using for each component the same analytic strategy.48
With the frequency of SD as the dependent variable, three
series of logistic regressions were successively performed
to assess the independent variables:
1) by subgroup of homogeneous variables, initial
logistic regressions to choose for each subgroup those
variables that best reflect changes in SD frequency, and
simultaneously identifying correlated variables;
2) two intermediate logistic regressions with the occu
pational and non–occupational variables selected from
the preceding step;
3) a final logistic regression with all the variables
selected at step 2 to stress the occupational factors
associated with SD, independently of the potential
non–occupational confounding factors.
The explanatory variables were coded so that the reference response was that presumed to favor a sound night's
sleep. The logistic regressions used a descending stepwise
procedure; the significance level at which variables entered
and exited the model was set at 10%.
Calculations were carried out on a UNIX RS6000 IBM
from the Centre de Ressources Informatiques of INSERM,
using the BMDP software.
SLEEP, Vol. 22, No. 4, 1999
Prevalence of SD in 1990 and in 1995
Globally, the prevalence of SD was 21.8% in 1990 and
24.6% in 1995. Among men, the prevalence of SD
increased from 19.1% to 21.0%, and among the women
from 25.7% to 29.4% (matched chi–2 p<0.05).
In 1990 as in 1995, the prevalence of SD increased with
age among men and women (p<0.001). In each generation,
at both dates (Table 2), the prevalence was higher among
women than men (p<0.001).
For each generation, for each sex, there is a trend toward
increased SD prevalence, even though the prevalences
observed in 1990 and in 1995 did not differ significantly.
Thus, the increase with age of SD frequency may not be
explained by generational effects.
In 1990, subjects lost to follow–up reported more SD
than subjects seen twice, at each age and for both sexes, but
these differences were not statistically significant.
There was no statistical generational effect, for either
sex, among the subjects seen again in 1995 and working at
that time. Nonetheless, among the no longer working men
in 1995, there were significant differences for the genera-
495
Age, Working Conditions, and Sleep Disorders—Ribet et al.
tions born in 1938 and 1943 (p<0.001 in both cases). But
this generational effect is perhaps attributable to their
non–working status, which for these generations essentially means that they lost their jobs. An identical phenomenon
was observed among the women without, however, being
statistically significant.
It must also be noted that in 1990 as in 1995, the prevalence of SD among the subjects no longer working in 1995
was greater than among the subjects who were still at work
in 1995 (p=0.01 for each sex in 1990 and 1995).
to hurry.
To be no longer working in 1995 was associated both
with an increase in incidence of SD and an increase in disappearance of SD.
We observe that the effect of a factor associated with the
disappearance of SD is strictly inverse to its effect on incidence: a 1990 factor associated with a lower rate of disappearance of SD in 1995 was also associated with a higher
incidence.
For only one factor, that is, whether the person was still
at work in 1995, the results for disappearance and incidence were not related in so simple a way. Thus, for subjects who did not report SD in 1990, the highest incidence
was among those who no longer worked in 1995. At the
same time, however, among subjects who suffered from SD
in 1990, the highest rate of disappearance in 1995 was also
observed among those no longer working in 1995.
We can also note that some occupational factors appear
to be related only to the incidence of SD but not to disappearance: age at first employment, work week often longer
than 48 hours, shift work, and exposure to vibrations.
In this context, it is interesting to note that in the factor
analysis for incidence, age is not associated with SD, but
remains associated with SD disappearance, insofar as the
likelihood of this disappearance decreases with age.
Moreover, in both incidence and disappearance analysis,
the sex factor remains associated with SD: incidence was
greater and disappearance was lower in women.
Incidence and disappearance of SD after the five–year follow–up
On the whole, the incidence of SD was 14.6%: 12.6%
among the men and 17.7% among the women, with a statistically significant difference between the sexes
(p=0.001). By generation, the incidence was significantly
lower among men than women (p=0.05 among those born
in 1938 and 1953, p=0.001 among those born in 1943 and
1948) (Table 3). For men, this incidence varied little
between the 1953 and 1948 cohorts but increased for those
of 1943 and 1938. Among the women, incidence increased
with age, stabilizing for the women born in 1938.
On the whole, 39.3% of the subjects with SD in 1990
had no SD in 1995: 42.4% of the men and 36.1% of the
women (p=0.01). For both sexes, this improvement rate
was highest for the subjects who were 37 years old in 1990.
It decreased until the age of 47 years, then increased slightly between 47 and 52 years. At the ages of 42 and 52 years,
significantly more men than women reported disappearance of SD.
While the finding that SD globally can disappear, except
for men born in 1948 and women born in 1938, matched
analysis showed that there were more new cases than cases
of disappearance (matched chi–2 p<0.05) for each sex and
for each generation.
DISCUSSION
Our study shows clear and strong links between age, sex,
and SD among employed persons older than 37 years. The
frequency of SD increases with age and is higher at all ages
among women workers. No generational effects were
observed.
Many independent risk factors were identified. Some are
work–related, some not, and others are related to health
conditions.
Occupational factors that aggravate SD are, in the first
place, age at first employment, shift work or work week
often longer than 48 hours, followed by exposure to vibrations. Alongside these objectifiable occupational factors,
psychosocial occupational factors pertaining to the perception of work also play a role; in particular, the judgment
that to have to hurry at work is difficult or irksome is associated with an increase in the incidence of SD.
These results support findings already noted in the literature and also open the way to new investigations. We must
nonetheless discuss the limits of our findings and underline
possible biases.
The ESTEV sample population includes only employees
followed by an occupational physician. To the extent that
the results may be applicable more widely, they could be
extended only to a population working principally in pri-
Multivariate analysis
Tables 4 and 5 show the values of the odds ratios (analysis of incidence and disappearance), adjusted by sex and
age, for the factors that were significantly associated with
SD in the bivariate analysis. Table 4 reports the objectifiable occupational factors and Table 5 the psychosocial
occupational factors.
Table 6 shows only the occupational factors resulting
from the final logistic regressions to analyze incidence and
disappearance of SD.
The occupational factors in 1990 that appeared to be risk
factors for incidence of SD were: age at first employment,
shift work, work week often longer than 48 hours, exposure
to vibrations, and obligation to hurry.
Two occupational factors were associated with the disappearance of SD by 1995: awkward postures and to have
SLEEP, Vol. 22, No. 4, 1999
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
other questions for appropriate assessment in the data base;
in particular, accident data were not recorded.
Finally, we must consider whether some bias in the relation between work and SD may be caused by possible confounding factors that were not assessed. Working conditions related to the organization of work and how the
employee perceives them were assessed by four
non–exhaustive factors. Other factors are sometimes mentioned in the literature, particularly interpersonal relations
with colleagues or supervisors.25,32 We could not study
these characteristics because they were not assessed in the
survey.
Our analysis also did not take into account some specific factors that have been noted as possible risk factors for
SD: exposure to organic solvents50 and, for women, premenstrual syndrome.21,25,51 The first factor was first suggested too late (1992) to be assessed it in the ESTEV survey. The second factor appears to us to be linked more to
anxious and depressive conditions, areas that we did not
take into account in the analyses to avoid a risk of
over–adjustment.
Also, some potential confounding factors have not been
taken into account, such as sleeping in a noisy environment
particularly for those living in an urban area. This factor
had been assessed only in 1995, and its relation with SD in
1995 adjusted for age and sex was not statistically significant. On the other hand, in this survey, there was no assessment of companion's sleep, but at no moment the variable
concerning the marital status was associated to incidence of
SD between 1990 and 1995.
Overall, any extension of these results, about either incidence or disappearance of SD, requires great caution and is
also limited by our method for assessing SD. Nonetheless,
the results about the relations between the factors examined, particularly those that were job–related, and SD
appear solid. The size of the sample population, the design
of the survey, and the small proportion of subjects lost to
follow–up minimize the possibility of artifacts.
Finally, by separating in time the assessment of factors
(from 1990) and the assessment of the effects (SD in 1995),
the "subject" bias has been lessened. It seems highly
unlikely that subjects responded to items about SD as a
function of their answers five years earlier to questions
about work–related variables.
vate industry and trade. Moreover, the survey did not
include subjects younger than 37 years or older than 52
years in 1990, making it applicable only to workers in the
second half of their careers. Finally, although the geographic regions surveyed cover the diversity of the socioeconomic composition of France, we cannot guarantee that
the sample population is representative of the entire French
working population.
It is, however, a random sample selected from an
exhaustive list of employees who were patients of the occupational physicians participating in this survey. The high
rate of participation (88% in 1990 and 87% in 1995), stable
for age and sex in all seven regions, limits the possible bias
from non–participation. Of those selected but not interviewed in 1990, 50%, irrespective of age and sex, refused
to participate.
Two main questions remain. First, the possibility of
selection bias that may have resulted from the voluntary
nature of the occupational physicians' participation. They
are, however, sufficiently numerous per region to cover a
wide spectrum of professions and economic activities. It is
thus highly unlikely that the employees questioned for the
final sample were selected for any relation to SD or any
other principal criterion. Second, the possibility of
response bias that may have resulted from subjects' appreciation about possible gain or decrement (on their employment) from their answers utilization. But again this is
unlikely, because all questionnaires used in the survey were
anonymous, and this fact had been clearly specified to each
potential participant who could accept or not to be enrolled
in the sample studied.
The Healthy Worker effect may, however, have played a
role in the selection of the sample population. In particular,
we might well wonder whether subjects in ill health were
still at work in 1995. The longitudinal study is one response
to this general problem, on condition that the percentage of
subjects lost to follow–up is not too high. This condition
was met here, with 13% lost to follow–up.
This percentage, moreover, is not constant with age, but
is highest in the 1938 generation, that is, for the oldest subjects. It is therefore possible that the relation between risk
factors and SD diminished because the drop–outs have a
higher rate of SD than the others at the start of the survey.
The assessment of SD has some disadvantages. Our
analysis, in effect, is based on perceived problems felt and
not objectivized.47 Nonetheless, it is generally agreed that
there is a correlation between polysomnographic data and
the subjective assessment of sleep.49 In addition, we must
note that the NHP indicator used to assess SD tries to pinpoint "serious" disorders, and we have further stiffened the
criterion by eliminating occasional symptoms as much as
possible. Moreover, we must recognize that the sleep
dimension of the NHP does not take into account what happens about daytime sleepiness. Futhermore, there are no
SLEEP, Vol. 22, No. 4, 1999
Interpreting these results
The estimated frequency of SD (with the definition
used) is similar to that found in other studies. Similarly, in
almost all of the published results, the relation of SD to age
and sex is observed. The quality and/or quantity of sleep
deteriorates with age, and women – whatever their age –
have more SD than men. Several authors have attributed
part of this difference to the greater prevalence of neurotic
disorders among women, but to the extent that neurotic
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Age, Working Conditions, and Sleep Disorders—Ribet et al.
problems may also well be the cause and not the consequence of SD, the question remains open for debate.19,52,53
Our study suggests that increased prevalence with age is
more likely associated with a lower rate of SD disappearance than to increased incidence with age. Taking into
account the many risk factors other than age associated
with the incidence of SD, our results suggest that age might
more play a role in maintaining SD rather than triggering
them.
Our results substantiate the findings already reported in
cross–sectional studies, that various occupational factors,
particularly shift work or rotating shift work,6,27,29,30,33–43
play a role in SD. Our longitudinal approach strengthens
these findings. This is also true for overtime work represented by a work week longer than 48 hours.25,45
Unlike other studies, however, we did not find any relation between SD and the time traveled between home and
work,26,54 exposure to loud noise,30,44 continued training,29
perceived interest in work,25 work diversification,25,29,31
and socioeconomic category.29 As far as the latter factor is
concerned, however, we can hypothesize that it is confounded in our study by the educational level, which was
associated with SD. Similarly, the fact that we did not find
an association with some factors pertaining to the perception of working conditions may result from colinearity
between these factors and the variables about working condition constraints which were put into the logistic regressions.
In the literature, besides age and sex, the non–professional factors that have been associated with an increase in
SD include number of children,29,32 low educational level,25
birth outside France,25 lack of sports activity,25,32,45,55,56 and
other physical or mental health problems.22,23,24,29,33
Conversely, one study found no evidence of a relation with
marital status.32 We must nonetheless bear in mind that
these results are essentially based on cross–sectional studies.
Our study suggests some directions that it should be
explored in specific studies. One area involves the working
conditions: employees were asked whether the conditions
concerned them and, if so, whether they found the work
bearable or irksome and difficult. The incidence of SD
appeared lowest among those who were concerned but
found their working conditions acceptable. Not being concerned by the problem was related to a higher incidence of
SD, and being concerned and finding the work irksome was
associated with the highest incidence. The apparent contradiction for the subjects not facing the problem might be
explained in terms of the psychodynamics of work: in a
normal working situation, one should be affected by the
demands of the work place or the profession exercized;
employees who are not concerned are either not following
some rules or hiding dysfunctions at work. Either case
could have somatic repercussions such as SD.
SLEEP, Vol. 22, No. 4, 1999
Moreover, certain factors, such as shift work, appear to
be associated with the incidence but not the disappearance
of SD. In other words, removing the factor (end of exposure) is not linked to a diminution in the condition. We
might interpret this result as indicating more chronic SD
among those exposed to this factor. Such a result reinforces
the likelihood of a causal relation and in any case is an
argument for the earliest possible prevention since the disappearance of SD becomes less and less likely with age.
Futhermore, the results have shown that age at first
employment was also related to an increase in SD. Thus, it
is possible that the age factor includes some effect related
to the number of working years, or that age at which the
person started working in conjunction with the occupational hazards, and that both might lead to SD. It is not possible to totally exclude these two hypotheses which can both
explain that incidence of SD between 1990 and 1995 was
not related to SD, but that age at first employment was.
Inversely, a bigger issue of age per se appeared in the study
of disappearance of SD which decreased with age during
the same period. The balance between incidence and disappearance led to an increase in prevalence with age. In an
additional study (not shown) it appeared that both age in
1990 and age at first employment were independent prognostic factors of the prevalence of SD in 1995.
Finally, the most surprising result involves differences
between those still at work at the five–year follow–up and
those who were no longer working. For the latter, the incidence of SD increased, but so did the frequency of disappearance. One hypothesis that might explain this contradiction is the following: among those who did not have SD in
1990, the loss of an occupational activity gave rise to disorders in the stability of their lives which then led to SD; on
the other hand, among those who suffered SD in 1990, the
loss of employment is also a loss of exposure to the associated risk factors, which could favor the disappearance of
SD. This hypothesis might be pursued by examining SD
frequency as a function of the subjects' reason for not working in 1995 (unemployment, retirement, or health status)
and age (or generation). An examination of the factors
assessed in 1990 shows that subjects no longer working in
1995 had been more exposed in 1990 than their still–working counterparts to: shift work, repetitive work under time
constraints, awkward postures, carrying heavy weights,
loud noise. In addition, unemployment (job loss) was the
predominant explanation of the non–working status in
1995, for both sexes, among subjects between 37 and 47
years of age in 1990 (67% among men, and 58% among
women).
Finally, a subsequent study could usefully consider not
only the state of the factors at a given moment but their
changes in the course of follow–up, to learn the role of
changes in the factors in relation to their initial state, but
also to investigate more thoroughly the possible causal role
502
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Conclusion
The results of these analyses allowed us to confirm that
age and sex are among the most important risk factors for
SD. Indeed, SD seem to be indicators that should not be
neglected in studies involving aging.
Alongside these factors, our study supports the idea that
occupational factors play an important role in the occurrence of SD. Among these factors, those linked to how
working conditions are experienced, especially the fact of
finding it difficult to have to hurry, play a role as important
as that of objective working conditions such as shift work
or work week often longer than 48 hours and exposure to
vibrations.
Health factors also have their own causal role (e.g., musculo–skeletal pain).
Summing up, the multiple causes of SD and probably of
the continuation of SD suggest that an approach that deals
specifically with risks is, from the viewpoint of possible
action, insufficient to diminish the prevalence of SD. Such
decrease would require primary prevention in the framework of a strategy that encompasses both the conditions
and organization of the work by reducing the level of standard risks, improved ergonomy of repetitive work, but also
taking into account the individual job paths, particularly
age at first employment and at intervention.
ACKNOWLEDGMENTS
We wish to thank all occupational physicians who participated in the ESTEV survey and particularly Dr Annie
Touranchet for her kind support in the carrying out of the
survey.
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