Short answer: A common reason is the dawn phenomenon: blood sugar rises in the early morning hours even without food. A 1985 study traced it mainly to overnight surges of growth hormone, which raise glucose production and lower glucose use. In 248 people with type 2 diabetes, the median rise from the overnight low to before breakfast was 16 mg/dL. The older idea that a night low causes a morning high (the Somogyi effect) has rarely held up in studies.
The dawn phenomenon is the early-morning rise in blood sugar, and in insulin needs, seen in many people with diabetes. In a 1985 study of 6 people with type 1 diabetes on a steady overnight insulin drip, plasma glucose rose from 98 mg/dL at midnight to 225 mg/dL at 8 a.m.; glucose production went up by 65% and glucose clearance fell by half.
When the researchers blocked the nightly growth hormone surges, the rise did not happen. When they added growth hormone back in pulses, it returned. Blocking adrenaline-type signals made no difference. They concluded that nightly growth hormone surges were mainly responsible.
| Study | People | Finding |
|---|---|---|
| Stephenson 1989 | 97 people with type 1 diabetes, 231 glucose profiles | A rise between 3 and 6 a.m. in 68% of profiles, averaging 56 mg/dL; rises over 50 mg/dL were less common on basal-bolus insulin (27%) than on conventional insulin therapy (41%) |
| Monnier 2013 | 248 people with type 2 diabetes not on insulin, on continuous glucose monitors | Median rise of 16 mg/dL from the overnight low to before breakfast; it added about 0.4% to HbA1c on average |
In the 2013 study, the size of the morning rise was similar whether people were on diet alone, metformin-type drugs, or drugs that make the pancreas release insulin. The authors wrote that none of the oral diabetes medicines then available eliminated it.
The Somogyi effect is the idea that a low blood sugar during the night triggers a rebound high by morning. Studies have mostly not found this:
So a low morning number is the stronger hint of a missed night low. See low blood sugar at night on insulin for the signs.
Readings in the middle of the night, or a continuous glucose monitor, show whether glucose rises steadily from a low point or drops low first, as in the studies above. Any change to insulin doses or timing is for the prescriber to decide. Dose Track can log basal insulin doses and their times next to glucose readings, so the overnight pattern is easy to bring to the appointment.
Read more on Dose Track: Blood sugar · Basal insulin · HbA1c · Hypoglycemia