Final answer:
To assess for Somogyi phenomenon, a nurse should monitor the adolescent's blood glucose levels at intervals throughout the night.
Step-by-step explanation:
To assess for Somogyi phenomenon in an adolescent with type 1 diabetes mellitus (DM), a nurse should monitor blood glucose levels during the night. This phenomenon is a response to hypoglycemia during the night with a subsequent morning rise in blood sugar. To accurately identify if the Somogyi effect is occurring, the nurse needs to check the blood glucose levels at intervals during the night when hypoglycemia is suspected, which can then lead to hyperglycemia as the body compensates by releasing glucagon and other hormones.