Sleep duration and quality in permanent night work: an observational field study.

PURPOSE: Permanent night workers conceivably show better adaptation of circadian rhythms to night work than shift workers and therefore better possibilities of obtaining sufficient sleep of good quality after night shifts. We investigated the effect of night shifts including number of consecutive shifts on sleep among self-selected permanent night workers, and studied if the effect of night shifts differed between morning and evening types and compared with 3-shift workers. METHODS: The study population included 90 permanent night workers followed for 14 days (warehouse workers, 1228 observation days, 80% males). For comparison, we included 70 3-shift workers followed for 26 days (police officers, 1774 observation days, 100% men). Total sleep time (TST), primary sleep duration (PSD), and sleep efficiency were assessed by actigraphy. Measures of sleep quality and diurnal type were self-reported. RESULTS: Among permanent night workers, TST, PSD, difficulties falling asleep, disturbed sleep, and the number of awakenings decreased after night shifts compared with days without night work. Sleep efficiency, difficulties awakening, and non-refreshing sleep increased. More consecutive night shifts were associated with shorter TST and PSD. Sleep outcomes did not differ by diurnal type. Permanent night workers had fewer awakenings after night shifts than 3-shift workers, but no other differences were observed. CONCLUSION: This study does not provide evidence that supports recommendation of permanent night work to reduce adverse effects of night shifts on sleep. A limited number of consecutive night shifts is recommended to reduce accumulation of sleep debt.

Maternal exposure to psychosocial job strain during pregnancy and behavioral problems in the 11-year-old children: a Danish cohort study.

Maternal psychosocial stress may impact child neurodevelopment, but little is known regarding psychosocial job strain. We hypothesized high psychosocial job strain during pregnancy was associated with behavioural problems in the 11-year-old children. Mothers in the Danish National Birth Cohort (1996-2002) were included if they worked, provided information on job strain [Karasek's model: high job strain (often job demand/seldom job control) and passive (seldom or sometimes job demands/seldom job control)] during early pregnancy. At the 11-year follow-up, children (N = 30,592), mothers (N = 30,993), and teachers (N = 12,810) responded to the Strength and Difficulties Questionnaire (SDQ), a screening tool for child behaviour. Scores for hyperactivity, conduct, emotional and peer problems were dichotomised [80% (no) vs. 20% (yes)] according to Danish norms (yes/no). Maternal job strain was not associated with behavioural problems with teachers as informants. When assessed by child or mother, high maternal job strain increased risk of child behavioural problems, but risks were more pronounced for mothers in passive jobs [maternal assessment of total difficulties/odds ratio (95% confidence interval): high strain-girls: 1.16 (0.97-1.40); boys: 1.24 (1.02-1.50). Passive girls: 1.43 (1.21-1.68); boys: 1.25 (1.05-1.49)]. This is one of the first studies on this topic. The different types of maternal job strain were partly associated with child behavioural problems at 11 years; more so if mothers worked in passive rather than the hypothesized high strain jobs. Findings showing dependency on informant could not only indicate unmeasured confounding or rater's bias, but also selection in the smaller numbers of teacher informants or different environments of interaction with the children.

Effort-reward imbalance at work and weight changes in a nationwide cohort of workers in Denmark.

OBJECTIVE: To investigate the relation between effort-reward imbalance (ERI) at work and subsequent weight changes. METHODS: We included participants from a population-based cohort of workers in Denmark (mean age = 47 years, 54% women) with two (n = 9005) or three repeated measurements (n = 5710). We investigated the association between (a) ERI (ie, the mismatch between high efforts spent and low rewards received at work) at baseline and weight changes after a 2-year follow-up (defined as ≥5% increase or decrease in body mass index (BMI) vs stable), and (b) onset and remission of ERI and subsequent changes in BMI. Using multinomial logistic regression we calculated risk ratios (RR) and 95% confidence intervals (CI), adjusted for sex, age, education, cohabitation, migration background, and follow-up time. RESULTS: After 2 years, 15% had an increase and 13% a decrease in BMI. Exposure to ERI at baseline yielded RRs of 1.09 (95% CI: 0.95-1.25) and 1.04 (95% CI: 0.90-1.20) for the increase and decrease in BMI, respectively. There were no differences between sex and baseline BMI in stratified analyses. The onset of ERI yielded RRs of 1.04 (95% CI: 0.82-1.31) and 1.15 (95% CI: 0.84-1.57) for subsequent increase and decrease in BMI. The RRs for the remission of ERI and subsequent increase and decrease in BMI were 0.92 (95% CI: 0.71-1.20) and 0.78 (95% CI: 0.53-1.13), respectively. Of the ERI components, high rewards were associated with a lower risk of BMI increase. CONCLUSION: ERI was not a risk factor for weight changes. Future studies may investigate whether this result is generalizable to other occupational cohorts and settings.

Night work and miscarriage: a Danish nationwide register-based cohort study.

OBJECTIVE: Observational studies indicate an association between working nights and miscarriage, but inaccurate exposure assessment precludes causal inference. Using payroll data with exact and prospective measurement of night work, the objective was to investigate whether working night shifts during pregnancy increases the risk of miscarriage. METHODS: A cohort of 22 744 pregnant women was identified by linking the Danish Working Hour Database (DWHD), which holds payroll data on all Danish public hospital employees, with Danish national registers on births and admissions to hospitals (miscarriage). The risk of miscarriage during pregnancy weeks 4-22 according to measures of night work was analysed using Cox regression with time-varying exposure adjusted for a fixed set of potential confounders. RESULTS: In total 377 896 pregnancy weeks (average 19.7) were available for follow-up. Women who had two or more night shifts the previous week had an increased risk of miscarriage after pregnancy week 8 (HR 1.32 (95% CI 1.07 to 1.62) compared with women, who did not work night shifts. The cumulated number of night shifts during pregnancy weeks 3-21 increased the risk of miscarriages in a dose-dependent pattern. CONCLUSIONS: The study corroborates earlier findings that night work during pregnancy may confer an increased risk of miscarriage and indicates a lowest observed threshold level of two night shifts per week.

Night work and sick leave during pregnancy: a national register-based within-worker cohort study.

OBJECTIVE: The aim of our study was to investigate the acute effect of night work during pregnancy on the risk of calling in sick the following day using register-based information and the workers as their own controls. METHODS: Using the payroll-based national Danish Working Hour Database, including all public hospital employees in Denmark, we identified 9799 pregnant women with ≥1 day shift and ≥1 night shift and ≥1 day of sick leave during the first 32 pregnancy weeks from January 2007 to December 2013. We performed fixed effects logistic regression, that is, within-worker comparisons, of the risk of sick leave of any duration starting within 24 hours after night shifts of different length versus day shifts. RESULTS: Most of the participants were nurses (64%) or physicians (16%). We found an increased relative risk of sick leave following night shifts compared with day shifts during all pregnancy trimesters. The risk was highest for night shifts lasting >12 hours (OR 1.37, 95% CI 1.15 to 1.63 for nurses; OR 1.87, 95% CI 1.69 to 2.08 for physicians) and among women aged >35 years (OR 1.42, 95% CI 1.24 to 1.63). CONCLUSION: Among Danish public hospital employees night shifts during pregnancy, especially shifts longer than 12 hours, increased the risk of calling in sick the following day independent of personal factors and time-invariant confounders in all pregnancy trimesters.

Night-time work and all-cause mortality in the general working population of Denmark.

PURPOSE: A recent study among female nurses in Denmark found an increased mortality among night-time workers, which has raised concerns about the sufficiency of the EU Working Time Directive. The aim of the present study was to examine the relationship between night-time work and all-cause mortality among full-time employees in the general workforce of Denmark. METHODS: Interview data from the Danish Labour Force Surveys, 1999-2013, were linked to national registers with individual-level data on occupation, industry, socioeconomic status (SES), emigrations and deaths. The participants (N = 159,933) were followed from the end of the calendar year of the interview until the end of 2014. Poisson regression was used to estimate rate ratios for all-cause mortality, with and without stratification by sex and socioeconomic status. A likelihood ratio test was used to test the overall null-hypothesis, which stated that the mortality rates were independent of night-time work, SES × night-time work and sex × night-time work. RESULTS: The likelihood ratio test did not reject the null hypothesis (p = 0.14). The rate ratio for all-cause mortality among employees with vs. without night-time work was estimated at 1.07 (95% CI 0.97-1.19) after adjustment for age, sex, SES, calendar time, weekly working hours and time passed since the start of follow-up. CONCLUSIONS: The present study did not find any statistically significant associations between night-time work and all-cause mortality among employees in the general workforce of Denmark.

Long working hours and stroke among employees in the general workforce of Denmark.

AIMS: A systematic review and meta-analysis have found that long working hours were prospectively associated with an increased risk of overall stroke. The primary aim of the present study was to test if this finding could be reproduced in a sample that has been randomly selected from the general workforce of Denmark. A secondary aim was to estimate the association for haemorrhagic and ischaemic stroke separately. METHODS: Individual participant data on 20- to 64-year-old employees were drawn from the Danish Labour Force Survey, 1999-2013, and linked to data on socio-economic status (SES), migrations, hospitalisations and deaths from national registers. The participants were followed from the time of the interview until the end of 2014. Poisson regression was used to estimate age-, sex- and SES-adjusted rate ratios for stroke as a function of weekly working hours. RESULTS: With 35-40 working hours per week as reference, the estimated rate ratios for overall stroke were 0.97 (95% confidence interval (CI) 0.83-1.13) for 41-48 working hours, 1.10 (95% CI 0.86-1.39) for 49-54 working hours and 0.89 (95% CI 0.69-1.16) for ≥55 working hours. The estimated rate ratios per one category increase in working hours were 0.99 (95% CI 0.93-1.06) for overall stroke, 0.96 (95% CI 0.88-1.05) for ischaemic stroke and 1.15 (95% CI 1.02-1.31) for haemorrhagic stroke. CONCLUSIONS: Our analysis does not support the hypothesis that long working hours are associated with increased rates of overall stroke. It suggests, however, that long working hours might be associated with increased rates of haemorrhagic stroke.

Socio-occupational class, region of birth and maternal age: influence on time to detection of cryptorchidism (undescended testes): a Danish nationwide register study.

BACKGROUND: Cryptorchidism (undescended testes) is associated with poor male fertility, but can be alleviated and fertility preserved to some degree by early detection and treatment. Here we assess the influence of socio-occupational class, geographical region, maternal age and birth cohort on time to detection and correction of cryptorchidism. METHODS: All boys born in Denmark, 1981 to 1987 or 1988 to 1994, with a diagnosis of cryptorchidism were identified in nationwide registers. The boys were followed for a diagnosis until their 16th birthday. The age at first diagnosis was noted and used as proxy for time to detection of cryptorchidism. Parental employment in the calendar year preceding birth was grouped into one of five socio-occupational classes. Geographical region was defined by place of birth in one of 15 Danish counties. Detection rate ratios of cryptorchidism were analyzed as a function of parental socio-occupational group, county, maternal age and birth cohort by use of Poisson regression. RESULTS: Some 6,059 boys in the early and 5,947 boys in the late cohort received a diagnosis of cryptorchidism. Time to detection was independent of parental socio-occupational group and maternal age but differed slightly between geographical regions. A similar pattern was obtained for surgical correction after a diagnosis. Age at diagnosis decreased by 2.7 years from the early to the late cohort. CONCLUSIONS: These results indicate that childhood socio-occupational inequality in detection and correction of cryptorchidism would play a negligible role in male infertility in a life course perspective. Geographical region may have exerted some influence, especially for the oldest cohort.

Validity of self-reported night shift work among women with and without breast cancer.

OBJECTIVES: This study aimed to estimate the validity of self-reported information on ever-night shift work among women with and without breast cancer and illustrate the consequences for breast cancer risk estimates. METHODS: During 2015-2016, 225 women diagnosed with breast cancer and 1800 matched controls without breast cancer employed within the Danish hospital regions during 2007-2016 participated in a questionnaire-based survey. Their reported night shift work status was linked with objective payroll register day-by-day working hour data from the Danish Working Hour Database and the Danish Cancer Registry. For the breast cancer patients and their matched controls, we estimated sensitivity and specificity for ever-working night shifts using the payroll data as the gold standard. We also used quantitative bias analysis to estimate the impact on relative risk estimates for a hypothetical population. RESULTS: For breast cancer patients, we observed a sensitivity of ever-night shifts of 86.2% and a specificity of never-night shifts of 82.6%. For controls, the sensitivity was 80.6% and the specificity 83.7%. Odds ratio for breast cancer in a hypothetical population decreased from 1.12 [95% confidence interval (CI) 1.03-1.21] to 1.05 (95% CI 0.95-1.16) when corrected by the sensitivity and specificity estimates. CONCLUSION: This study shows that female breast cancer patients had slightly better recall of previous night shift work than controls. Additionally, both breast cancer patients and controls recalled previous never-night shift work with low specificity. The net effect of this misclassification is a small over-estimation of the relative breast cancer risk due to night shift work.

Night and evening shifts and risk of calling in sick within the next two days - a case-crossover study design based on day-to-day payroll data.

OBJECTIVE: Night and evening work is associated with risk of sickness absence, but little is known about the acute effects of these types of shifts on sickness absence. The aim of the current study is therefore to examine the risk of calling in sick within two days after a night or an evening shift. METHODS: By use of a case-crossover design, odds of calling in sick within two days after a night or an evening shift compared to day shifts were analyzed within the same person. Day-to-day information on shifts and sickness absence were derived from the Danish Working Hour Database on 44 767 cases. Data were analyzed using conditional logistic regression. The analyses were supplemented by extensive testing of methodological choices. RESULTS: Analyses showed higher odds of calling in sick after a night shift [odds ratio (OR) 1.22, 95% confidence intervak (CI) 1.14-1.30] and lower odds after an evening shift (OR 0.89, 95% CI 0.84-0.93) compared to day shifts within the same person. Testing of methodological choices suggested that in particular the duration of case and control periods, time between these periods along with the number of control periods affected the results. CONCLUSION: This large and unique within-person study among Danish hospital employees indicate that the risk of calling in sick is affected by the types of shifts, independently of sex, age, and time-invariant confounding. Extensive testing identified important methodological choices eg, length and number of included periods to consider when choosing the case-crossover design.

Trajectories of marginal part-time work and risk of depression. Does job or income insecurity mediate the relation?

OBJECTIVES: Working few hours a week, known as marginal part-time work, may increase both job and income insecurity, which have been linked to the risk of depression. This study examines the association between marginal part-time work and depression and the mediating role of job and income insecurity. METHODS: We included 30 523 respondents of the Danish Labor Force Survey (DLFS) between 2010 and 2017 and linked them to register-based information on weekly working hours, which was used to identify full-time workers and model group-based trajectories of marginal part-time. These data were linked with survey information on job and income insecurity, and register-based information on hospital-diagnosed depression or redeemed anti-depressant drugs in the following two years. We estimated hazard ratios (HR) by Cox proportional hazards models and conducted mediation analyses to estimate the natural direct and indirect effects using job and income insecurity as mediators. RESULTS: We identified three distinct trajectories of marginal part-time work: constant marginal part-time work, mobile towards marginal part-time work, and fluctuating in and out of marginal part-time work. Compared with full-time workers, the constant [HR 2.42, 95% confidence interval (CI) 1.83-3.20], mobile (HR 2.84, 95% CI 2.16-3.75), and fluctuating (HR 3.51, 95% CI 2.07-5.97) trajectories all had higher risks of depression. There was no evidence of mediation by either job (HR 1.02, 95% CI 0.92-1.12) or income (HR 0.98, 95% CI 0.89-1.08) insecurity. CONCLUSIONS: We found a higher risk of depression following marginal part-time work. The higher risk was not mediated by job or income insecurity.

Night shift work characteristics and risk of incident coronary heart disease among health care workers: national cohort study.

BACKGROUND: Night work has been associated with coronary heart disease. The present study examined exposure-response relations between quantitative night work characteristics and coronary heart disease (angina pectoris or myocardial infarction) with the aim to contribute to evidence-based recommendations for low-risk night work schedules. METHODS: We followed 100 149 night workers (80% women) and 153 882 day workers (78% women), all health care workers in Denmark with day by day payroll information on night shifts from 2007 to 2015. We analysed data with Poisson regression stratified by sex and adjusted for age, calendar year, diabetes, family history of cardiovascular disease, educational level, occupation, indicators for obesity, hypercholesterolaemia, and hypertension. RESULTS: Female and male night workers worked on average 1.7 and 1.8 night shifts per month for an average duration of less than 4 years. During follow-up, 1198 night and 2128 day workers were hospitalized with first-time coronary heart disease. When compared with day workers, the overall incidence rate ratios for female and male night workers were 1.06 [95% confidence interval (CI): 0.97, 1.17] and 1.22 (95% CI 1.07, 1.39). Highest risks were observed in top exposure categories for several night work characteristics. However, no consistent exposure-response relations by number of monthly night shifts, cumulative night shifts, years with rotating night shifts, years with any night shift and consecutive night shifts were observed among the night workers of either sex. CONCLUSIONS: This study of a population with low exposure to night work does not indicate that reducing extent of monthly night shifts, cumulative night shifts, years with rotating night shifts, years with any night shift and consecutive night shifts would reduce the risk of coronary heart disease.

Maternal life and work stressors during pregnancy and asthma in offspring.

BACKGROUND: Maternal stressors during pregnancy are potential risk factors for asthma in offspring. However, previous studies have been limited by the use of self-reported data focusing on stressors either in private life or at work. This study examined the association between maternal stressors both in private life and at work during pregnancy and asthma in offspring. METHODS: In the Danish National Birth Cohort, 75 156 live-born singletons born during 1996-2002 were identified. Maternal information on job title were available around weeks 12-16 of gestation. Data on maternal bereavement, life-threatening illness, suicide attempt and alcohol or drug abuse of a close relative and offspring childhood asthma (3-10 years of age) were obtained from Danish nationwide registers. Maternal psychosocial work stressors (job control, psychological job demands, emotional job demands, work-related violence and threats of work-related violence) were estimated by the use of job-exposure matrices. The association between maternal stress and childhood asthma was analysed in Cox models adjusted for maternal age, comorbidity and parity. RESULTS: Neither private-life nor work stressors were related to onset of asthma in offspring. Separate analyses by parental atopy or onset of asthma in offspring supported the main findings. CONCLUSIONS: This study does not support an elevated risk of childhood asthma related to exposure to stress during pregnancy.

Working time characteristics and long-term sickness absence among Danish and Finnish nurses: A register-based study.

BACKGROUND: Working time regimes in Denmark and Finland share many similarities such as nursing personnel working in highly irregular shift systems. Yet, there are also differences for example in policy on when and how the employers are compensated for sickness absence. OBJECTIVE: We aimed to investigate the association between different working hour characteristics and long-term sickness absence and whether these associations differed within various age groups in two large datasets of nursing personnel from Denmark and Finland. DESIGN: Based on objective payroll data we used Poisson regression models to calculate incidence rate ratios with 95% confidence intervals to prospectively assess the risk of long-term sickness absence in relation to annual working hour characteristics. The analyses were adjusted for age, sex, short-term sickness absence, and weekly working hours. SETTING(S): Danish and Finnish nursing personnel. PARTICIPANTS: 31,729 Danish and 6970 Finnish nursing personnel with ≥ 0.5 Whole-Time Equivalent, registered in the database ≥ 1 year, 18-67 years of age with less than 30 days sickness absence in baseline year 2008. METHODS: Working hour characteristics were assessed for 2008: time of day; day; evening; night. Duration of shift; long shifts (9-12 h); very long shifts (12-24 h); quick returns (< 11 h between two shifts); long weeks (> 40 h/week); very long weeks (> 48 h/week); and consecutive night shifts (≥ 5 night shifts). Long-term sickness absence was assessed as first incidence of 30 or more consecutive days off in 2009-2015. RESULTS: The Danish data showed having evening work or five or more consecutive night shifts were associated with higher risk of long-term sickness absence. When excluding pregnant women, night work was also associated to higher risk of sickness absence. When stratifying on age groups, we observed a lower risk of sickness absence in the youngest age groups and a higher risk among the oldest. The Finnish results showed a higher risk of sickness absence when working nights, longs shifts, quick returns, and long work weeks. When stratifying on age groups, the results showed similar tendencies as the Danish. CONCLUSIONS: The results show that the scheduling of working hours is likely to affect the risk of long-term sickness absence and that the risk differs in different age groups. No consistent picture was found for the results from Denmark and Finland. Differences may be due to contextual differences thus comparison of risk of sickness absence in relation to working hours between countries should be performed with caution. Tweetable abstract: A recent study from Denmark and Finland shows higher risk for long sickness absence among nurses with five or more consecutive night shifts.

How to schedule night shift work in order to reduce health and safety risks.

Objectives This discussion paper aims to provide scientifically based recommendations on night shift schedules, including consecutive shifts, shift intervals and duration of shifts, which may reduce health and safety risks. Short-term physiological effects in terms of circadian disruption, inadequate sleep duration and quality, and fatigue were considered as possible links between night shift work and selected health and safety risks, namely, cancer, cardio-metabolic disease, injuries, and pregnancy-related outcomes. Method In early 2020, 15 experienced shift work researchers participated in a workshop where they identified relevant scientific literature within their main research area. Results Knowledge gaps and possible recommendations were discussed based on the current evidence. The consensus was that schedules which reduce circadian disruption may reduce cancer risk, particularly for breast cancer, and schedules that optimize sleep and reduce fatigue may reduce the occurrence of injuries. This is generally achieved with fewer consecutive night shifts, sufficient shift intervals, and shorter night shift duration. Conclusions Based on the limited, existing literature, we recommend that in order to reduce the risk of injuries and possibly breast cancer, night shift schedules have: (i) ≤3 consecutive night shifts; (ii) shift intervals of ≥11 hours; and (iii) ≤9 hours shift duration. In special cases - eg, oil rigs and other isolated workplaces with better possibilities to adapt to daytime sleep - additional or other recommendations may apply. Finally, to reduce risk of miscarriage, pregnant women should not work more than one night shift in a week.

Long weekly working hours and ischaemic heart disease: a follow-up study among 145 861 randomly selected workers in Denmark.

OBJECTIVES: The aim of the present study was to test if incidences of ischaemic heart disease (IHD) and usage of antihypertensive drugs are independent of weekly working hours (WWH) among full-time employees in Denmark. DESIGN AND PARTICIPANTS: Data on WWH from participants of the Danish labour force surveys, 1999-2013, were linked on an individual level to national registers with data on socioeconomic status (SES), industry, emigrations, redeemed prescriptions, hospital contacts and deaths. Participants were followed until the end of 2014 (on average 7.7 years). Poisson regression was used to model incidence rates as a function of WWH. The analyses were controlled for calendar time, time passed since start of follow-up, employment in the healthcare industry, age, sex, SES and night work. RESULTS: In total, we found 3635 cases of IHD and 20 648 cases of antihypertensive drug usage. The rate ratio of IHD was 0.95 (95% CI 0.85 to 1.06) for 41-48 compared with 32-40 WWH and 1.07 (0.94 to 1.21) for >48 compared with 32-40 WWH. The corresponding rate ratios for antihypertensive drug usage were 0.99 (0.95 to 1.04) and 1.02 (0.97 to 1.08). No statistically significant interactions between WWH and sex, SES and night work, respectively, were found. CONCLUSION: In this Danish sample, we did not find any statistically significant association between WWH and IHD or antihypertensive drug usage.

Payroll data based description of working hours in the Danish regions.

The aim was to describe the organization of working hours in the Danish regions according to sex, age and calendar year. Based on the Danish Working Hour Database (DWHD), individuals were classified according to schedules: Permanent day (57.8%), evening (1.7%), or night (1.2%); day/evening (22.0%); day/night (6.6%); evening/night (0.6%); and day/evening/night (10.2%). More men (9.1%) than women (5.9%) worked day/night, whereas more women (10.9%) than men (7.4%) worked day/evening/night. More young than older employees worked day/evening/night, and fewer worked permanent day or night. From 2008 to 2015 we observed a trend towards more employees working permanent day and fewer employees working other schedules. Altogether DWHD provides a strong tool in research on working hours.

Combined exposure to lifting and psychosocial strain at work and adverse pregnancy outcomes-A study in the Danish National Birth Cohort.

BACKGROUND: Previous studies have investigated physical and psychosocial job exposures separately in relation to foetal growth. We therefore investigated if occupational lifting and psychosocial job strain interact to affect foetal growth and gestational length. We hypothesised that heavy lifting and high job strain would increase the risk of impacted foetal growth (small or large for gestational age) and preterm birth. METHODS: The cohort included 47,582 pregnancies from the Danish National Birth Cohort (1996-2002), where the woman was pregnant at 22 gestational weeks (GW), expected one child and worked ≥30 hours/week. Information on occupational lifting and psychosocial job strain was derived from an interview (16±3.0 GW). Data to calculate small and large for gestational age (SGA/LGA) and gestational length was retrieved from the Medical Birth Register. Interaction between lifting and job strain (Karasek's model) was analysed by multinomial logistic regression. RESULTS: Overall, the adjusted regression analysis showed statistically significant interaction between lifting and job strain for SGA and LGA. For each additional 250 kg lifted/day, high strain women (high Demand/low Control) had increased odds of giving birth to a LGA-child (OR = 1.15; 95% CI 1.06-1.26), whereas women in the active group (high Demand/high Control) had increased odds of giving birth to a SGA child (OR = 1.12; 95% CI 1.03-1.23). When women lifting ≤1000 kg/day were excluded in the sensitivity analyses the interaction between lifting and job strain became insignificant. No interaction of lifting and job strain was found for gestational length. CONCLUSIONS: The main findings may give some support to our hypothesis, as lifting in combination high with job strain increased the risk of giving birth to a LGA child. This finding was, however, not supported in the sensitivity analysis and no association of the interaction was found relative to gestational length.

Working Time Arrangements as Potential Risk Factors for Ischemic Heart Disease Among Workers in Denmark: A Study Protocol.

BACKGROUND: It has long been suspected that a worker's risk of developing an ischemic heart disease (IHD) may be influenced by his or her working time arrangements. A multitude of studies have been performed, and special attention has been given to long working hours and nighttime work. The statistical powers of the individual studies have, however, generally been too low to either dismiss or confirm an actual relationship, and meta-analyses of underpowered studies are generally associated with publication bias. Hence, uncertainty remains and whether these factors indeed are related to IHD has yet to be settled. OBJECTIVE: This project will test whether the incidences of IHD and usage of antihypertensive drugs among employees in Denmark are independent of weekly working hours and nighttime work. The objective of this paper is to present the intended analyses. METHODS: We will link individual participant data from the Danish labor force survey, 1999-2013, to data on socioeconomic status, industry, emigrations, redeemed prescriptions, hospitalizations, and deaths from registers covering the entire population of Denmark. The study will include approximately 160,000 participants, who will be followed through the registers, from the time of the interview until the end of 2014, for first occurrence of IHD and for antihypertensive drug treatment. We will use Poisson regression to analyze incidence rates as a function of nighttime work and of weekly working hours. RESULTS: We expect results to be ready in mid-2017. CONCLUSIONS: To our knowledge, this will be the largest study ever of its kind. It will, moreover, be free from hindsight bias, since the hypotheses, inclusion criteria, significance levels, and statistical models will be completely defined and published before we are allowed to link the exposure data to the outcome data.

How do different definitions of night shift affect the exposure assessment of night work?

The aim is to show how different definitions affect the proportion of shifts classified as night shifts. The Danish Working Hour Database was used to calculate number of night shifts according to eight definitions. More than 98% of the total night shifts were night shifts by use of both the reference definition (at least 3 h of work between 24:00 and 05:00) and definitions using a period during the night. The overlap with definitions based on starting and ending time was less pronounced (64-71 %). The proportion of classified night shifts differs little when night shifts are based on definitions including a period during the night. Studies based on other definitions may be less comparable.