In this study of middle-aged men and women, working more than 55 hours per week was associated with lower scores on 2 of the 5 tests of cognitive function. Long working hours at baseline were related to poorer performance on the vocabulary test at both baseline and follow-up. Furthermore, long working hours predicted decline in performance on the reasoning test over a 5-year follow-up period. These effects were robust to adjustments for 20 potential confounding factors, such as education, occupational position, physical diseases (cardiovascular dysfunction), psychosocial stress factors, sleep problems, and health risk behaviors.
We found an association between long working hours and decline in the scores for the AH 4-I reasoning test and associations with the Mill Hill vocabulary tests at baseline and at follow-up. The AH 4-I test is also recognized as a measure of fluid intelligence, that is, executive function or “meta” cognitive ability as it integrates other cognitive processes such as memory, attention, and speed of information processing. Fluid intelligence is seen to be intrinsically associated with information processing and involves short-term memory, abstract thinking, creativity, ability to solve novel problems, and reaction time. It is the aspect of intelligence most affected by aging, biologic factors, diseases, and injuries (52
). Fluid intelligence usually increases up to the mid-20s, after which it gradually declines until the 60s when a more rapid decline takes place.
The Mill Hill vocabulary test measures crystallized intelligence that is assumed to accumulate during the lifespan through education, occupational and cultural experience, and exposure to culture and intellectual pursuits (52
). Crystallized abilities usually increase up to the sixth or seventh decade of age and may not decrease until after 80 years of age. We found the Mill Hill scores to remain relatively stable as expected for this middle-aged cohort. However, the Mill Hill scores were lower among employees with long working hours at both baseline and follow-up. This consistency with 2 separate measures with a 5-year interval suggests not only a plausible finding but also stability of the far-reaching effect of long working hours on vocabulary. We did not find an interaction effect between follow-up employment status and working hours on significant outcomes, which suggests that the associations found are not dependent on employment status at follow-up. However, people who work long hours might be exposed to a narrower variation of intellectual pursuits, that is, only to those that are related to their work tasks, and therefore might not be able to develop a wide variety of functions in crystallized intelligence measured by the test. However, reversed causality is also possible: Employees with lower cognitive ability may be more prone to work overtime than workers with good cognitive ability in order to get their work done.
Previous literature, mostly cross-sectional, suggests that long working hours are associated with various health outcomes, the strongest effects being observed for cardiovascular diseases, fatigue, and sleep disturbances (2
). These can also be hypothesized to be mediating mechanisms for the association between long working hours and cognitive decline. Hypertension is associated with cognitive dysfunction by producing subtle disturbances in cerebral perfusion and affecting brain cell metabolism (19
). However, we found no evidence of an association between long working hours and hypertension or coronary heart disease, suggesting that the effect of long hours on cardiovascular dysfunction, if any, is unlikely to explain cognitive decline in this study.
Another hypothesis on mediating mechanisms links long working hours with psychological stress and poor recovery from work as indicated by sleeping problems and reduced sleep. Psychological stress has been suggested as affecting the brain via 2 neuroendocrine systems: 1) the sympathetic adrenomedullary system with the secretion of epinephrine and norepinephrine and 2) the hypothalamic-pituitary-adrenocortical system with the secretion of cortisol (54
). Of the few studies in the field, only 1 study has found an association between long working hours and neuroendocrinologic stress markers (55
). We found that long working hours were associated with short sleep duration and psychologic distress but not with sleep disturbances. Further adjustment for these factors did not provide support for the hypothesis that psychological distress and poor recovery act as mediating mechanisms.
The third hypothesis suggests that long working hours may affect cognitive function through health risk behaviors. Evidence on the association between long working hours and unhealthy behaviors is weak, but there is stronger evidence for the relation between health behaviors and cognitive function (22
). We found that adjustment for all these health risk behaviors had no effect on the association between long working hours and cognitive function, suggesting that health risk behaviors may not be an important mediating or confounding variable.
When working hours were entered into the model as a continuous variable, we found an association between long hours and better phonemic fluency at baseline but not at follow-up. This inconsistency is also reflected in the lack of an association between the categorical working hours and phonemic fluency. More research is needed to determine whether employees with long working hours do better than other employees on tests of verbal fluency. Out of 15 analyses, we found 2 statistically significant interaction effects between working hours and sex, and sex-stratified analysis showed that long working hours were associated with poorer vocabulary performance among men but not among women. However, further research with larger samples is needed to examine potential sex differences in the association between working hours and cognition.
Strengths and limitations
The strengths of this study include a large sample size and the possibility to explore prospectively the association between long working hours and a possible change in cognitive function over a 5-year interval, which has not been feasible in earlier studies. Furthermore, we used 5 separate measures of cognitive function, allowing associations with specific aspects of cognition to be observed, and we were able to adjust for a large number of covariates as potential confounding or mediating factors between the exposure and outcome.
There are also important limitations in this study. First, the period of 5 years for cognitive decline might not be sufficient to detect a significant decline in cognitive function in general. Second, the Whitehall II cohort is based on civil servants and not representative of the entire working population, limiting the generalizability of our results. Third, we used self-reported working hours, with inherent problems of recall. Fourth, middle-aged occupational cohorts, such as ours, are subject to a healthy survivor effect as the study design involves participants who are employed and gradually excludes those who develop work disability. However, all cohort studies focusing on work-related exposures at midlife are open to health-related selection because participants need to be employed. Because poor health is linked with worse cognition, the healthy survivor effect is likely to lead to conservative estimates of the associations found. The baseline of the present study was approximately 15 years after inclusion into the Whitehall II study; men, employees in the higher occupational grades, and those free from coronary heart disease were slightly overrepresented. However, the associations among work hours, vocabulary, and reasoning were robust to adjustments for sex, occupational grade, and health. Furthermore, the similarity of these associations in the complete case and multiple imputation analyses suggests that loss to follow-up after the baseline is an unlikely source of bias in this study.
Decline in cognitive function has already been shown to be present among the middle aged (9
). As mild cognitive impairment predicts dementia (10
) and mortality (56
), the identification of risk factors for mild cognitive impairment in middle age is important. The results of this study show that long working hours may be one of the risk factors that have a negative effect on cognitive performance in middle age. Our findings can have clinical significance, as the 0.6- to 1.4-unit difference in aspects of cognitive functioning between employees working long hours and those working normal hours is similar in magnitude to that of smoking, a risk factor for dementia (59
), which has been found to affect cognition in the Whitehall II study (60
). However, further research is needed to identify the potential underlying factors for the relation between long working hours and cognitive function and to examine the generalizability of our findings.