The circadian timing system coordinates daily cycles in physiological functions, including glucose metabolism and insulin sensitivity. However, how continuous enteral nutrition interacts with the daily cycles in glucose control in critically ill patients in the Intensive Care Unit (ICU) is unknown. Therefore, the goal of this retrospective, observational study was to characterize 24-h variation in blood glucose values in critically ill patients during continuous enteral feeding and to explore the influence of potential sources of bias. To this end, all time-stamped glucose measurements and other relevant clinical variables from adult ICU patients who stayed in the ICU for at least 4 days and received enteral tube feeding during their stay were extracted from the MIMIC-IV clinical database. Linear mixed-effects modelling was used to determine the effect of time of day on blood glucose values during continuous enteral feeding, adjusted for relevant confounders. Sensitivity and subgroup analyses were carried out to explore potential sources of bias. In total, 208,154 glucose measurements were available from 6,944 ICU patients while they received enteral tube feeding. Time of day had a significant effect on blood glucose levels (p < 0.0001), with a peak of 9.61 [9.56–9.67; estimated marginal means, 95% CI] mmol/L at 10:00 in the morning and a trough of 8.72 [8.67–8.78] mmol/L at 03:00 at night. Subgroup analyses showed that 24-h variation in glucose levels was present regardless of diabetes diagnosis. These results indicate that glucose levels show marked 24-h variation during continuous feeding, even in ICU patients. This daily pattern persists when controlling for potential sources of bias, suggesting that the observed 24-h pattern may be due to biological variation. The sharp rise in blood glucose levels during the early morning is reminiscent of the dawn phenomenon, which occurs as a result of reduced insulin sensitivity at the time of highest endogenous glucose production and may reflect increased insulin resistance frequently observed in the ICU population.