It refers to somewhat mysterious higher-than-expected blood sugars in the early morning hours, as between 6 an 9 AM. Even if you don’t eat breakfast; that’s why it seems mysterious. Assuming you slept all night, why would your blood sugars be too high, even before breakfast?
Dr. Richard Bernstein talks about it at DiabetesHealth. Dr Bernstein is a great resource for folks with diabetes, and to physicians who treat diabetes.
Peter at Hyperlipid suggests that dawn phenomenon isn’t as simple and straightforward as Dr. Bernstein says. In fact, Peter seriously questions, Dr. Bernstein’s explanation that it’s caused by the liver removing too much insulin from circulation. For those who don’t click through, I’ll summarize by saying Peter found evidence that nighttime growth hormone elevations (this is normal) lead to lipolysis (breakdown of body fats into free fatty acids to be used by various tissues for energy) and the FFA’s cause a degree of insulin resistance, leading to higher blood sugar levels in the early morning.
As a practical side note, some people with diabetes (should I start using PWD’s?) seem to suppress Dawn Phenom by drinking alcohol at bedtime.