There are two reasons why your blood sugars levels may be high in the morning – the dawn phenomenon and the Somogyi effect.
Dawn phenomenon.
The dawn phenomenon is the end result of a combination of natural body changes that occur during the sleep cycle and can be explained as follows. Your body has little need for insulin between about midnight and about 3:00 a.m. (a time when your body is sleeping most soundly). Any insulin taken in the evening causes blood sugar levels to drop sharply during this time. Then, between 3:00 a.m. and 8:00 a.m., your body starts churning out stored glucose (sugar) to prepare for the upcoming day as well as releases hormones that reduce the body’s sensitivity to insulin. All of these events happen as your bedtime insulin dose is also wearing off. These events, taken together, cause your body’s blood sugar levels to rise in the morning (at "dawn").
Somogyi effect.
A second cause of high blood sugar levels in the morning might be due to the Somogyi effect (named after the doctor who first wrote about it). This condition is also called "rebound hyperglycemia." Although the cascade of events and end result – high blood sugar levels in the morning – is the same as in the dawn phenomenon, the cause is more "man-made" (a result of poor diabetes management) in the Somogyi effect. There are two potential causes. In one scenario, your blood sugar may drop too low in the middle of the night and then your body releases hormones to raise the sugar levels. This could happen if you took too much insulin earlier or if you did not have enough of a bedtime snack. The other scenario is when your dose of long-acting insulin at bedtime is not enough and you wake up with a high morning blood sugar.