BACKGROUNDGestational diabetes mellitus is associated with an increased risk of maternal, fetal, and neonatal morbidities. Chronobiological disorders have recently been identified as risk factors for those morbidities. The disorders include chrononutritional disorders related to meal frequency and content according to the sleep-wake cycle, sleep disorders related to sleep quality, and chrono-obesity disorders, such as abnormal weight gain because of sleep deprivation and time of eating. OBJECTIVEThis study aimed to assess whether a chrononutritional and sleep hygiene intervention can improve maternal glycemic control and reduce the proportion of large-for-gestational-age newborns among women with gestational diabetes mellitus. STUDY DESIGNThis randomized controlled trial included 103 women with gestational diabetes mellitus who were carrying a singleton fetus and assigned to either the intervention group (n=33) or the control group (n=70). The intervention group was assigned to a chrononutrition and sleep hygiene program, in addition to the usual care for gestational diabetes mellitus, from the time of diabetes mellitus diagnosis to birth, whereas the control group received the usual gestational diabetes mellitus care. RESULTSThe chrononutritional and sleep hygiene intervention significantly reduced the proportion of women with suboptimal glycemic control (<80% of the plasma glucose values at target), after adjustment for maternal age, prepregnancy body mass index, gravidity, history of gestational diabetes mellitus, and large for gestational age (relative risk, 0.28; 95% confidence interval, 0.18–0.81). The effect of the intervention on balancing maternal glycemic control was mainly because of the decreased carbohydrate intake in the evening interval of the day (relative risk, 0.8; 95% confidence interval, 0.64–0.99). However, the intervention had no effect on the proportion of large-for-gestational-age newborns. CONCLUSIONThe chrononutritional and sleep hygiene intervention can improve maternal glycemic control.