'Dawn phenomenon' refers to:

A more common cause of pre breakfast hyperglycemia is the waning of the evening or bedtime insulin and/or the dawn phenomenon. That is, reduced tissue sensitivity to insulin between 5am and 8am (dawn), due to spikes of growth hormone released hours before, at onset of sleep. For hyperglycemia due to waning of overnight basal insulin and/or dawn phenomenon, an increase in the evening dose of the basal insulin or shifting it from dinneime to bedtime (or both) can be effective. Ref: German M.S. (2011). Chapter 17. Pancreatic Hormones and Diabetes Mellitus. In D.G. Gardner, D. Shoback (Eds), Greenspan's Basic & Clinical Endocrinology, 9e.