Night work, long working hours, psychosocial work stress and cortisol secretion in mid-life: evidence from a British birth cohort.
OBJECTIVE
To examine the relationships between exposure to workplace factors (night work, extended working hours, psychosocial work stress) and cortisol secretion, and to test whether workplace factors interact, resulting in combined effects.


METHODS
Multiple linear and logistic regression was used to test relationships between workplace factors and cortisol secretion in the 1958 British birth cohort at 45 years. Salivary cortisol was measured twice on the same day to capture the post-waking decline, facilitating the analysis of different cortisol patterns: (1) time 1 (T1, 45 minutes post-waking); (2) time 2 (T2, 3 h after T1); (3) average 3 h exposure from T1 to T2 cortisol; and (4) T1 to T2 change. To identify altered diurnal cortisol patterns we calculated: (1) flat T1-T2 change in cortisol; (2) top 5% T1; (3) bottom 5% T1; and (4) T1 hypo-secretion or hyper-secretion. Models were adjusted for socioeconomic position at birth and in adulthood, qualifications, marital status, dependent children, and smoking status.


RESULTS
25% of men and 8% of women were exposed to >1 workplace factor (night work, extended work hours, job strain). Night work was associated with a 4.28% (95% CI 1.21 to 7.45) increase in average 3 h cortisol secretion independently of job strain or work hours. Night workers not exposed to job strain had elevated T1 cortisol (5.81%, 95% CI 1.61 to 10.19), although for T2 cortisol it was night workers exposed to low job control who had elevated levels (11.72%, 95% CI 4.40 to 19.55). Men (but not women) working >48 h/week had lower average 3 h cortisol secretion (4.55%, 95% CI -8.43 to -0.50). There were no main effects for psychosocial work stress. All associations for T2 and average 3 h cortisol secretion weakened slightly after adjustment for confounding factors, but associations for T1 cortisol were unaffected by adjustment.


CONCLUSIONS
Our study suggests that night work in particular is associated with elevated cortisol secretion and that cortisol dysregulation may exist in subgroups with specific combinations of stressors.
Thomas
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MRC Centre of Epidemiology for Child Health, UCL Institute of Child Health, London WC1N 1EH, UK. c.thomas@ich.ucl.ac.uk
Hertzman
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Power
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D002940:Circadian Rhythm / Q000502:physiology; D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D016272:Occupational Health; D012463:Saliva / Q000378:metabolism; D013315:Stress, Psychological / Q000378:metabolism / Q000503:physiopathology*; D013997:Time Factors; D014940:Work Schedule Tolerance / Q000502:physiology*
D006854:Hydrocortisone