We analyzed 216 overnight blood glucose profiles (samples at 2100, 0300, and 0700 h) in 75 consecutive patients with diabetes mellitus primarily to assess the impact of nocturnal hypoglycemia on morning hyperglycemia and secondarily to assess the frequency and magnitude of a night-to-morning increase in glucose levels in a clinical context. A dawn phenomenon (an 0300 to 0700 h increment in blood glucose) was rather uncommon in our patients (about one-third of profiles), was readily demonstrable in groups of patients only when nocturnal glucose levels were low (≤50 mg/dl) or normal (51–100 mg/dl), and was generally not of great magnitude (mean 0700 h glucose levels of 114 mg/dl after 0300 h values of ≤100 mg/dl). Nocturnal hypoglycemia (0300 h blood glucose of ≤50 mg/dl, 7% of profiles) was followed by significant increments in blood glucose. However, the 0700 h glucose values averaged only 113 mg/dl and ranged up to only 172 mg/dl. Clearly, the magnitude of this Somogyi phenomenon was not great. Mean glucose levels were not higher at 1100, 1600, or 2100 h the day after nocturnal hypoglycemia than those at the same times the day before hypoglycemia. Thus, nocturnal hypoglycemia does not commonly result in major morning, or daytime, hyperglycemia in patients with diabetes mellitus sampled while using their usual therapeutic regimens.