Metformin/sitagliptin - What the cause of high blood sugar in the morning?

Morning Hyperglycemia. An elevated blood glucose level on arising in the morning is caused by an insufficient level of insulin, which may be caused by several factors:
1.the dawn phenomenon,
2. the Somogyi effect, or
3. insulin waning.

The dawn phenomenon is characterized by a relatively normal blood glucose level until approximately 3 AM, when blood glucose levels begin to rise. The phenomenon is thought to result from nocturnal surges in growth hormone secretion, which create a greater need for insulin in the early morning hours in patients with type 1 diabetes. It must be distinguished from insulin waning (the progressive increase in blood glucose from bedtime to morning) and from the Somogyi effect (nocturnal hypoglycemia followed by rebound hyperglycemia). Insulin waning is frequently seen if the evening NPH dose is administered before dinner; it is prevented by moving the evening dose of NPH insulin to bedtime. It may be difficult to tell from a patient’s history which of these causes is responsible for morning hyperglycemia. To determine the cause, the patient must be awakened once or twice during the night to test blood glucose levels. Testing at bedtime, at 3 AM, and on awakening provides information that can be used to make adjustments in insulin to avoid morning hyperglycemia.