Shift work and circadian misalignment

Shift work — any schedule displacing working hours outside the conventional 07:00–18:00 window, including fixed night shifts and rotating patterns — chronically misaligns behavioral cycles (sleep, feeding, activity) with the endogenous ~24-hour circadian clock governed by the suprachiasmatic nucleus (SCN). Nocturnal light suppresses melatonin, inverts the cortisol rhythm, and shifts meal timing into biologically nocturnal hours, creating internal desynchrony between central and peripheral clocks. A controlled forced-desynchrony protocol (Scheer et al., PNAS 2009) showed that even a few days of misalignment elevated mean arterial pressure, raised postprandial glucose and insulin, and lowered leptin in healthy adults — changes corresponding to prediabetes and hypertension trajectories if sustained. In the UK Biobank cohort (Yang et al., JAHA 2022; n ≈ 36,900), hypertensive participants who worked night shifts usually or always had a 16% higher risk of cardiometabolic multimorbidity (concurrent diabetes, coronary heart disease, or stroke) versus day workers. The IARC classified night shift work as a Group 2A probable human carcinogen in 2007 (Monograph 98), reaffirmed in 2019 (Monograph 124), citing limited epidemiological signals for breast, prostate, and colorectal cancers plus mechanistic evidence from animal models. Cancer causation in humans remains associational; the metabolic and cardiovascular burden from circadian disruption is mechanistically well-supported and directly relevant to accelerated aging phenotypes.