This patient should measure her blood glucose level at 3 AM. The etiology of fluctuating fasting glucose values in diabetes can be multifactorial, including overnight hypoglycemia, dawn phenomenon, or inadequate insulin doses. To maintain normal blood glucose levels upon rising, an early morning physiologic release of catecholamines, cortisol, and growth hormone occurs to stimulate endogenous glucose production from the liver. Overnight hypoglycemia caused by overtreatment of diabetes or prolonged effects of recent physical exertion can lead to low-normal fasting glucose values and amplify the release of catecholamines, cortisol, growth hormone, and glucagon to increase endogenous glucose production, which can lead to hyperglycemia.
The Somogyi effect, or rebound hyperglycemia, describes elevated morning blood glucose levels resulting from release of compensatory hormones as described in response to prolonged nocturnal hypoglycemia. Although this mechanism was previously generally accepted, several small studies published in the late 20th century suggested that nocturnal hypoglycemia may not be a common cause of fasting morning hyperglycemia in patients with diabetes treated with insulin, although follow-up studies to confirm these findings have not been performed. Therefore, the clinical significance of rebound hyperglycemia has not been fully elucidated and the existence of the effect is not accepted by all physicians. However, a 3 AM blood glucose check would provide valuable information regarding this patient's overnight blood glucose pattern useful in informing changes to the patient's insulin regimen.
With the dawn phenomenon, fasting hyperglycemia occurs in the setting of inadequate basal insulin coverage to maintain the endogenous glucose value within a normal range. Food intake in the evening can also contribute to fasting hyperglycemia if it is inadequately covered with insulin. Overnight hypoglycemia and the dawn phenomenon can be distinguished by measuring the glucose level at 3 AM. Medications that affect the overnight glucose level need to be decreased if the 3 AM glucose level is low. Medications that affect the overnight glucose should be increased or added if the 3 AM glucose level is elevated.
Fast-acting insulin such as insulin lispro at bedtime increases the risk of hypoglycemia.
Metformin will decrease gluconeogenesis from the liver and improve fasting hyperglycemia. However, for similar reasons, overnight hypoglycemia must be excluded before this treatment could be safely initiated.
Increasing the insulin glargine dose could also worsen overnight hypoglycemia if that is the cause of the fasting hyperglycemia.
Despite the hemoglobin A1c level of less than 7%, the etiology of the patient's fasting hyperglycemia should be investigated. Detection of overnight hypoglycemia would necessitate immediate changes in her insulin regimen or food intake regardless of her hemoglobin A1c value.