Abstract 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 randomised 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; BMI 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% carbohydrate, 33% fat, 17% protein and 23 g fibre. Following an adaptation night, participants completed four simulated nightshifts, with cortisol levels measured ~hourly with additional measurements at 30, 60, and 120-minutes post-consumption of a meal, a snack or no-meal at 00:30 h. Mixed-effects ANOVAs analysed 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 (AUCg 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.
Grosser et al. (2025) studied this question.