Morning hyperglycaemia in diabetic subjects may be caused by the dawn phenomenon, or the Somogyi effect, or poor glycaemic control. The dawn phenomenon occurs when endogenous insulin secretion decreases or when the effect of the exogenous insulin administered to the patient the day before disappears, together with a physiological increase in insulin-antagonistic hormones. The Somogyi effect is present in the case of excessive amounts of exogenous insulin. The dawn phenomenon is more common than the Somogyi effect. To diagnose these phenomena, it is useful to measure plasma glucose levels for several nights between 3 a.m. and 5 a.m. or use a continuous glucose monitoring system. Although their treatment differs, the best way of preventing both the dawn phenomenon and the Somogyi effect is an optimal diabetes control with insulin therapy.