Studies have shown that sleep in shift workers is often displaced and/or shortened, particularly in connection with morning and night shifts. Several laboratory studies confirm that such sleep disturbances alter the regulation of several hormones - including cortisol, prolactin and growth hormone - in both shift/night workers as well as other populations. Workplace studies have shown that working nights and being on-call are not only related to increased adrenalin release, but also to reduced levels of testosterone.
A New Study
While the main negative effects of shift work are well documented, not all individuals are affected equally. In an attempt to better understand the variation in tolerance of shift work, researchers have focused on the endocrine differences between two types of shift workers, satisfied and dissatisfied. Their results show that the dissatisfied shift workers accumulated problems with sleep and fatigue across a work period.
The study, entitled "Hormonal Changes in Satisfied and Dissatisfied Shift Workers Across a Shift Cycle," was conducted by John Axelsson, Torbjörn Åkerstedt and Göran Kecklund, all of the National Institute for Psychosocial Factors and Health, Stockholm, Sweden; Anne Lindqvist of the Karolinska Institutet, Stockholm, Sweden; and Reine Attefors, of Hyltehalsan, Hyltebruk, Sweden. Their findings appear in the online section "Articles in Press" of the Journal of Applied Physiology. The journal is one of 14 scientific journals published monthly by the American Physiological Society (APS) (www.the-aps.org).
A total of 317 of 368 full time shift workers working as control room operators, shift engineers, machinists and shift supervisors at a paper and pulp factory in Sweden filled out a questionnaire where they rated their "satisfaction toward their work hours" (1 = very dissatisfied, 5 = very satisfied). These ratings were used to divide the shift workers into two groups that were either satisfied or dissatisfied with their shift schedule. The shift system had been in use for more than 20 years.
A total of 62 male shift workers were invited to participate in two health examinations where blood was drawn. Eventually, the study group included 42 men, of which 22 were satisfied (mean age of 44 years) and 20 dissatisfied (mean age 41 years). Relatively few shift workers were dissatisfied with their work hours; therefore, the resulting difference between satisfied and dissatisfied shift workers was 5.0±0 vs. 2.4±2.
The shift cycle (comprising 35 days and 5 shift teams) rotated rapidly and included 7 work periods (triads) and one week off. A "work period" consisted of three shifts during four days, with 8 hours off between the night- and afternoon-shift, and 9 hours off between the afternoon- and morning-shift. Each shift worker was examined twice, at the beginning of the morning shift during the first and the last of the seven work periods.
Fasting blood samples were obtained between 0700h and 0900h on the morning-shift during the first and the last (7th) work period and examined. The data were analyzed using repeated-measures analysis of variance (ANOVA) with 1 between-group and 1 within-group measurements.
The investigators found that:
- Dissatisfied shift workers had lower morning testosterone levels than satisfied ones, but the two groups did not significantly differ with respect to levels of cortisol or prolactin.
- The only significant change across the shift cycle was the lowering of morning levels of cortisol on the last morning shift in the shift cycle.
- In light of the testosterone results, they hypothesize that the low levels of testosterone may be due to disturbed sleep and/or a high sleep need, which in turn may result in more sleepiness and a greater need for recovery after a work period, and as a consequence, dissatisfaction with the shift schedule.
The results suggest that dissatisfaction with the shift system is related to lower testosterone levels, and that the latter are related to disturbed sleep/wakefulness and an increased need for sleep and recovery. In addition, working an entire shift cycle was related to a decrease of morning cortisol, which may relate to an adaptation to long-term stress.
Low testosterone levels and disturbed sleep might be key factors for developing shift intolerance, mainly by reducing the capacity to recover from shift work. Studies using larger sampling procedures are necessary to shed further light on the mechanisms involved in intolerance to shift work.
Source: Journal of Applied Physiology (Articles in Press)
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.