Circulating cortisol levels naturally follow a circadian rhythm. The influence of shift work on the hypothalamic–pituitary–adrenal (HPA) axis has been identified as a potential mechanism through which circadian desynchrony may lead to ill-health ( Nader et al., 2010). Controlled laboratory studies, designed to mimic shift work sleep/wake disruption, have demonstrated disruptions to the normal 24-h cycle of a number of metabolic, autonomic and endocrine system indicators ( Griefahn and Robens, 2010 Ribeiro et al., 1998 Scheer et al., 2009). Adverse health consequences are thought to arise as a result of chronic misalignment between endogenous circadian timing systems and behavioural sleep/wake and feeding cycles ( Ruger and Scheer, 2009). Large-scale prospective cohort studies have found that rotating shift work is associated with increased risks of weight gain ( Suwazono et al., 2008), diabetes ( Pan et al., 2011), heart disease ( Fujino et al., 2006), stroke ( Brown et al., 2009) and some cancers ( Kubo et al., 2006). The most common shift pattern in the UK is a two-shift double-day system, consisting of early and late day shifts (such as 6 am–2 pm and 2 pm–10 pm), with workers alternating on a routine basis ( Steel, 2011). In industrialised countries, almost one in five workers participates in shift work, in which different groups of workers replace each other in the same role ( ILO, 2004). ![]() Early shift days were associated with significantly higher levels of circulating cortisol during waking hours than late shifts or rest days. Both types of work shift were associated with more stress, tiredness and lower happiness than rest days, but statistical adjustment for mood ratings did not alter the findings. Early shifts were also associated with shorter sleep duration but co-varying for sleep duration did not alter the effects of shift on the cortisol rhythm. Early shifts were associated with a higher cortisol increase in response to awakening (CAR i), a greater total cortisol output over the day (AUC G) and a slower rate of decline over the day than late shifts or rest days. Cortisol responses were analysed with repeated measures analysis of variance with shift condition (early, late, rest) and sample time (1–6) as within-subject factors. Waking time, sleep duration, sleep quality and working hours were also recorded. Sampling was scheduled at waking, waking + 30 m, waking + 2.5 h, waking + 8 h, waking + 12 h and bedtime. Pilots sampled saliva and completed subjective mood ratings in a logbook 6 times over the day on two consecutive early shift days, two late days and two rest days. The standard rotating shift pattern consisted of 5 early shifts (starting before 0600 h), followed by 3 rest days, 5 late shifts (starting after 1200 h) and 4 rest days. ![]() ![]() Participants were 30 healthy male non-smoking pilots, mean age 39.4, employed by a short-haul airline. We aimed to investigate how early and late work shifts influenced the diurnal cortisol rhythm using a within-subjects study design.
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