Epidemiological studies indicate that shift-workers have an increased risk of type 2 diabetes mellitus (T2DM). Glucose tolerance and insulin sensitivity both are dependent on the circadian timing system (i.e., the time-of-day) and fasting duration, in rodents as well as humans. Therefore, question is whether manipulation of the circadian timing system, for example by changing the timing of feeding and fasting, is a potential preventive treatment for T2DM. Time-restricted feeding (TRF) is well-known to have profound effects on various metabolic measures, including glucose metabolism. However, experiments that directly measure the effects of TRF on glucose tolerance and/or insulin sensitivity at different time points throughout the 24 h cycle are lacking. Here we show, in rats, that TRF in line with the circadian timing system (i.e., feeding during the active phase) improves glucose tolerance during intravenous glucose tolerance tests (ivGTT) in the active phase, as lower insulin levels were observed with similar levels of glucose clearance. However, this was not the case during the inactive phase in which more insulin was released but only a slightly faster glucose clearance was observed. Contrasting, TRF out of sync with the circadian timing system (i.e., feeding during the inactive phase) worsened glucose tolerance, although only marginally, likely because of adaptation to the 4 week TRF regimen. Our results show that TRF can improve glucose metabolism, but strict adherence to the time-restricted feeding period is necessary, as outside the regular eating hours glucose tolerance is worsened.

Objective Epidemiological studies show that shift workers are at increased risk for type 2 diabetes. As modern societies increasingly require shift work, it seems crucial to determine whether there are long‐lasting health effects of rotational shifts. Methods This study examined the after‐effects of 4 weeks of time‐restricted feeding (TRF) during the light period (= light‐fed) in rats, an animal model for shift work. This study also included a TRF‐dark (= dark‐fed) control group. The aligned and misaligned feeding times of light and dark feeding are associated with poor and good health outcomes, respectively. Several physiological measures were monitored continuously; blood, liver, brown adipose tissue, and soleus and gastrocnemius muscle were collected following 11 days of ad libitum (AL) feeding after ending the TRF. Results In the dark‐fed animals, the day/night differences in food intake, activity, and respiratory exchange ratio were still enhanced at the end of the experiment. Light‐fed animals displayed the smallest day/night differences for these measures, as well as for body temperature. In both the light‐ and dark‐fed animals, rhythms in plasma glucose, nonesterified fatty acids, and gene expression had not fully recovered after 11 days of AL feeding. Importantly, the effects on gene expression were both tissue and gene dependent. Conclusions Our data indicate that rotational shift workers may have an increased risk of long‐lasting disturbed rhythms in several physiological measures after a period of shift work. Clearly, such disturbances may harm their health.