Our findings suggest that shorter leucocyte rTL, a putative marker of biological ageing, is associated with current employment and some characteristics of work schedule, specifically current and long-term full-time hours. Observed differences were not explained by a variety of covariates, including potential intermediates in pathways mediating schedule effects on health, such as obesity, smoking, perceived stress and other factors associated with rTL in this sample and other studies.6, 11, 14, 15
Although we did not directly evaluate schedule-related stress, the impact of current full-time or longer work hours on rTL was most apparent in women with higher epinephrine and perceived stress, implying a potential role of neuroendocrine stress responses. As the first to describe cross-sectional associations of current and past work schedule with a leucocyte rTL, these findings should be interpreted with caution but support further investigation of work and schedule-related stress in relation to rTL, especially in the context of prospective data and more specific information on work and schedule-related stress exposures.
Work schedule is a measure of chronic stress distinct from job strain, which relates to demands and control within the work-place. Work schedule may be an intrinsic stressor, or act indirectly through behaviours or confiicts relating to schedule control and demands of work and other roles. We saw no evidence of behavioural factors mediating schedule-related rTL differences; rather, our findings may suggest more direct schedule-related differences or something about the balance of work and other roles. Women currently working part-time and those with more years of part-time work had a significantly longer rTL. In a national sample, part-time work was related to lower job–family interference and job stress compared with full-time.16
The impact of full-time work on rTL differences was also most apparent in women with more children and earlier age at first birth, perhaps pointing towards stress arising from the need to balance work and other roles. More direct information on role strain was not available, and we had no data on demands of informal (unpaid) caregiving for children and adult relatives, common experiences among mid-life women.
In these cross-sectional data, we cannot account for self-selection out of different schedule types depending on work ability and career trajectory; a healthy survivor effect may be expected among women who maintain or start overtime positions over their worklife course. We saw no additional impact of current or more total years of overtime schedule. We had limited opportunities to disentangle the effects of overtime from full-time and part-time work. Stress from longer workhours may depend on predictability or control, compensation and personal fulfilment. Despite associations with job stress and perceived ‘overwork,’ a national survey found that having an overtime schedule was related to greater opportunities for developing abilities, participation in decision-making and job satisfaction.16
In some professions, longer hours are related to self-directed, fiexible work hours,17
and the impact of overtime may be buffered by resources enabling rest and recovery, or lower job strain.18
In this sample, women reporting current overtime schedules were more likely to have a graduate or professional education (27% vs 18% of those who worked full-time) and higher job control scores in a later job-strain questionnaire (not shown).
Although current employment was associated with shorter telomeres, women with a minimal work history (employed less than 25% of their potential workspan) had a significantly shorter rTL. This may reflect a positive impact of employment on health, or selection of unhealthy individuals out of the workforce.19
Post-hoc analyses showed that retired women had a significantly longer rTL than homemakers (age-adjusted rTL=5972 vs 5567 bp, p=0.023), which suggests resiliency from schedule-related stress after retirement. A recent study described improved self-reported health with retirement,20
though this may depend on conditions at work. Homemakers may differ from both employed and retired women, and have a unique set of stressors. Being non-employed has been associated with increased mortality and coronary heart disease in women,21, 22
but findings comparing homemakers with employed women are not consistent.23
Gender roles may influence the impact of work schedule-related stresses. Consistent with our findings, one study suggested somewhat longer telomeres in retired compared with currently employed men and also described significantly shorter telomeres in those who were unemployed.24
We had too few unemployed women to replicate this second finding. Future research should also consider rTL in relation to underemployment and precarious employment in both men and women.
Menopause is associated with female (ovarian) ageing, though published findings on telomere length, menopause and post-menopausal hormone use are inconclusive.25, 26
In the present study sample, menopausal status was not independently associated with rTL overall or in women aged 45–54 years (not shown). We observed differences by menopausal status in schedule-associations with rTL, but these were sometimes diminished in multivariate models. The meaning of work may vary by age, and work schedule-related stress might increase in older women if retirement or part-time work is not an option, especially in the context of other age-related changes in personal health or caregiving demands.
Interpreting cross-sectional data on work and health is challenging, owing to the cumulative impact of work on health, including development of chronic diseases that may lead to disability or early retirement. We considered current health status and behaviours, but did not account for past behaviours or changes over time. Longitudinal findings in a large occupational cohort showed that socio-economic status effects on mortality were substantially attenuated after adjusting for health behaviours measured several times over two decades.27
In the present study, similar patterns of work-related rTL differences were seen in analyses excluding those with heart disease, diabetes or poor health, which may be related to past health behaviours. We cannot rule out selection biases; women with the greatest work/life demands or chronic health problems may not have participated in the study, owing to the time and effort required.
A strength of this study is that lifetime job history and schedules were collected separately from stress or health-behaviour data. Errors in data recall are expected to be non-differential with respect to rTL but could be affected by age or education. Although our findings are limited by the lack of direct work-related stress measures, differences were most clearly seen at higher epinephrine levels in the presence of higher perceived stress. This suggests that physiological stress response may play a role. Although transiently influenced by acute exposures, overnight urinary catecholamine levels are an integrative measure of sympathetic nervous system activation spanning hours to days.28
We cannot explain why we saw limited differences stratifying by norepinephrine or cortisol, as these analytes are biologically related and correlated in these data.11
Cortisol has been studied extensively in relation to work-related stress.29
Typically characterised through circadian patterns and response to acute stress, interpretation of cortisol levels in first morning urines may be complicated by long-term stress-related changes in cortisol response. Our findings are also limited by a wide variation in assay performance, especially for norepinephrine and cortisol, and low specimen volume prohibited assay replication. This non-differential misclassification may have limited our ability to see potential underlying differences.
Other explanations may underlie our findings, including reasons for working (eg, personal fulfilment, economic necessity) or not working (eg, health, social and economic opportunity), schedule control and resources for balancing non-work demands (eg, household help and childcare).30
Though not adjusted for multiple comparisons, exploratory analyses suggest that the impact of work schedule on the rTL might be buffered by higher education or income, pointing towards the possible role of the type of work or socio-economic resources. Lower socio-economic status has been associated with mortality risk,31, 32
but associations with shorter rTL have been inconsistently observed.24, 33
Long-term work in multiple jobs was also associated with a shorter rTL, suggesting that economic necessity or strain in managing complex schedules might be important. The sample was limited to women not currently working rotating shifts; however, several aspects of shiftwork may act as schedule-related stressors (eg, work at night, rotating shifts). We saw a somewhat shorter rTL for long-term (>10 years) work at night or rotating shiftwork. Together, long-term work in multiple jobs, shift-work or work at night was associated with a shorter rTL in postmenopausal women, though this was attenuated with covariate adjustment (not shown).
Our findings of independent rTL differences for both current and past full-time work may reflect a short-term impact on leucocyte rTL and more lasting effects operating through different mechanisms. Interpretation of rTL associations typically implies unidirectional shortening with age and other exposures, as most somatic tissues do not normally express the telomere maintenance enzyme, telomerase. Some blood cells require telomerase as part of a normal immune response34
; there may be short-term and, to some extent, reversible effects of stress on leucocyte rTL, owing to changes in telomerase expression,35
as well as longer-term effects of cellular replication, oxidative stress and impact on the haematopoetic stem cell rTL. Longitudinal studies describe the maintenance or extension of leucocyte telomeres in up to a third of individuals sampled over time (eg, 8–10 years),36, 37
suggesting potential recovery from acute or chronic telomere shortening. An intensive, 3-month intervention in patients with prostate cancer showed an improvement in psychological distress associated with increased telomerase expression.38
Thus, both short- and long-term changes in stressors and resiliency may influence telomere length across the lifespan. Further research is needed to understand the public health implications of these findings. Longer work hours have been infrequently studied in women,9
though one study did show significantly elevated mortality associated with five overtime hours per week after 24 years of follow-up.39
Our study sample, drawn from a volunteer cohort of women at increased risk of breast cancer, was enriched for higher perceived stress, non-white race and smoking; replication is needed in other samples. However, the magnitude of schedule-related rTL differences in the present study was similar to the differences observed for established risk factors and consistent with other telomere research. Though cautious interpretation is warranted, findings could have widespread implications, as employment is a common and often extended lifetime experience, with related stresses arising at the interface of resources and demands, and possible links to socio-economic gradients in health.