STUDY OBJECTIVES: Cortisol regulates various physiological systems and exhibits a circadian rhythm influenced by sleep-wake and light-dark cycles. The cumulative effects of consecutive night shifts and nighttime eating on cortisol dynamics are not well understood. This sub-study of a larger randomized controlled trial aimed to explore these relationships.
METHODS: This laboratory study employed a three-arm, controlled, parallel design. Fifty-two healthy non-shift workers (age 24.5 ± 4.8 years; body mass index 24 ± 2.8 kg/m2) were assigned to one of three nighttime conditions: meal (n = 17), snack (n = 16), or no-meal (n = 19) at 00:30 h. Macronutrient content for the meal and snack was similar, comprising ~50 per cent carbohydrate, 33 per cent fat, 17 per cent protein, and 23 g fiber. Following an adaptation night, participants completed four simulated nightshifts, with cortisol levels measured ~hourly with additional measurements at 30, 60, and 120-min post-consumption of a meal, a snack, or no-meal at 00:30 h. Mixed-effects analysis of variances analyzed changes in cortisol levels resulting from nighttime eating and the effects of consecutive nightshifts.
RESULTS: Eating at night significantly influenced cortisol secretion, resulting in higher total cortisol output in the meal and snack conditions (area under the curve ground: p = .019 and p = .005), respectively, compared to the no-meal condition. Four consecutive nightshifts induced a temporal shift in the cortisol rhythm, with levels at 20:00 h on night 4 significantly elevated compared to night 1 (p = .007), and levels at 05:30 h significantly reduced on night 4 relative to night 1 (p = .003).
CONCLUSIONS: Nightshifts and eating during the nightshift disrupt the cortisol rhythm. Repeated disruptions may have cumulative effects, potentially impacting cortisol-sensitive tissues and increasing risk of significant health disorders.
CLINICAL TRIAL: Effects of night eating during simulated shift work on metabolism, cognitive performance, and mood in healthy adults. Australian New Zealand Clinical Trials Registry (ACTRN12616001556437). Statement of Significance This study represents the first investigation into the effects of consecutive simulated night shifts and altered meal timing on cortisol levels. Circadian rhythms are vital for regulating sleep-wake cycles and hormone secretion. Shift work disrupts the natural 24-h light-dark exposure, significantly impacting circadian regulation of behavioral, cellular, metabolic, and physiological processes. Our findings indicate that working night shifts and consuming food during these hours disrupts cortisol rhythms over consecutive nights. This research enhances the understanding of how circadian and behavioral disruptions collectively affect cortisol regulation. Additionally, these insights are crucial for developing effective strategies that address sleep, dietary practices, and eating habits for individuals frequently assigned to night shifts.
