Exploring circadian and meal timing impacts on cortisol during simulated night shifts.

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.