There are 3 possible reasons for high fasting blood glucose of 9.4 mmol/L after insulin administration in diabetes mellitus: insufficient nighttime insulin application; dawn phenomenon; and the Sumuje effect. 1. Inadequate nocturnal insulin application can reduce fasting blood glucose by increasing the dose of insulin at bedtime. It is necessary to monitor the blood glucose changes at night; 2. Dawn phenomenon: i.e., good glycemic control at night and no hypoglycemia occurs, only transient hyperglycemia occurs at dawn, which may be due to excessive secretion of free cortisol, growth hormone, etc. in the early morning; 3. Sumuje effect: hypoglycemia had occurred at night but was not noticed during sleep, due to the increased secretion of hormones antagonizing insulin in the body, which in turn caused rebound blood glucose elevation. In order to further clarify the cause of high fasting blood glucose in the morning, it is necessary to monitor blood glucose at 0:00, 2:00, 4:00, 6:00, and 8:00 at night under the supervision of a physician. Diabetic patients should pay attention to the changes of blood glucose value in the course of glucose-lowering treatment, especially also pay attention to the occurrence of hypoglycemia phenomenon, which should be recognized and dealt with at an early stage.