Fig. 1. Conceptual scheme for meal timing effects in shift work settings, study aims, and experimental design.

(A) Top: Night work, whereby meals typically occur at night, is hypothesized to misalign central and peripheral circadian oscillators. Bottom: Night work, whereby meals occur during the day, is aimed to align central and peripheral circadian oscillators (internal circadian alignment). (B) In this study, we tested whether endogenous circadian rhythms of metabolic markers (glucose and insulin) can be entrained to an FD 28-hour sleep/wake and 28-hour fasting/eating cycles, resulting in a misalignment between the central circadian pacemaker and metabolic organs, and in impaired glucose tolerance. We tested whether restricting food intake to the daytime (on a 24-hour cycle) during a 28-hour sleep/wake cycle maintains internal circadian alignment and appropriate glucose tolerance. (C) The 14-day laboratory study design presented as relative clock time (for a participant whose habitual wake-up schedule was 7 a.m.). Participants were randomized to the Nighttime meal control (NMC) group or the Daytime meal intervention (DMI) group (see Methods for details on the study design). Meals consumed during the FD protocol (including the identical test meals) are included in the scheme (see text for detailed timings). Isocaloric snacks were consumed hourly during the CR protocols.