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Context: Shift work is a risk factor for diabetes. The separate effects of the endogenous circadian system and circadian misalignment (ie, misalignment between the central circadian pacemaker and 24-hour environmental/behavioral rhythms such as the light/dark and feeding/fasting cycles) on glucose tolerance in shift workers are unknown. Objective: The objective of the study was to test the hypothesis that the endogenous circadian system and circadian misalignment separately affect glucose tolerance in shift workers, both independently from behavioral cycle effects. Design: A randomized, crossover study with two 3-day laboratory visits. Setting: Center for Clinical Investigation at Brigham and Women's Hospital. Patients: Healthy chronic shift workers. Intervention: The intervention included simulated night work comprised of 12-hour inverted behavioral and environmental cycles (circadian misalignment) or simulated day work (circadian alignment). Main Outcome Measures: Postprandial glucose and insulin responses to identical meals given at 8:00 am and 8:00 pm in both protocols. Results: Postprandial glucose was 6.5% higher at 8:00 pm than 8:00 am (circadian phase effect), independent of behavioral effects (P = .0041). Circadian misalignment increased postprandial glucose by 5.6%, independent of behavioral and circadian effects (P = .0042). These variations in glucose tolerance appeared to be explained, at least in part, by different insulin mechanisms: during the biological evening by decreased pancreatic β-cell function (18% lower early and late phase insulin; both P ≤ .011) and during circadian misalignment presumably by decreased insulin sensitivity (elevated postprandial glucose despite 10% higher late phase insulin; P = .015) without change in early-phase insulin (P = .38). Conclusions: Internal circadian time affects glucose tolerance in shift workers. Separately, circadian misalignment reduces glucose tolerance in shift workers, providing a mechanism to help explain the increased diabetes risk in shift workers.

Effects of the Internal Circadian System and Circadian Misalignment on Glucose Tolerance in Chronic Shift Workers