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.

Shift work is a risk factor for inflammation, hypertension and cardiovascular disease. This increased risk cannot be fully explained by classic risk factors. Shift workers’ behavioral and environmental cycles are typically misaligned relative to their endogenous circadian system. However, there is little information on the impact of acute circadian misalignment on cardiovascular disease risk in shift workers, independent of differences in work stress, food quality and other factors that are likely to differ between night and day shifts. Thus, our objectives were to determine the independent effect of circadian misalignment on 24-h high-sensitivity C-reactive protein (hs-CRP; a marker of systemic inflammation) and blood pressure levels—cardiovascular disease risk factors—in chronic shift workers. Chronic shift workers undertook two 3-day laboratory protocols that simulated night work comprised of 12-hour inverted behavioral and environmental cycles (circadian misalignment) or simulated day work (circadian alignment), using a randomized, crossover design. Circadian misalignment increased 24-h hs-CRP by 11% (P<0.0001). Circadian misalignment increased 24-h systolic blood pressure (SBP) and diastolic blood pressure (DBP) by 1.4 mmHg and 0.8 mmHg, respectively (both P≤0.038). The misalignment-mediated increase in 24-h SBP was primarily explained by an increase in SBP during the wake period (+1.7 mmHg; P=0.017), whereas the misalignment-mediated increase in 24-h DBP was primarily explained by an increase in DBP during the sleep opportunity (+1.8 mmHg; P=0.005). Circadian misalignment per se increases hs-CRP and blood pressure in shift workers. This may help explain the increased inflammation, hypertension and cardiovascular disease risk in shift workers.

Chronic shift work causes night shift intolerance, which is probably linked to the degraded plasticity of the circadian control system.