It refers to periodic episodes of hyperglycemia occurring in the early morning hours in DM patients. The dawn phenomenon has been documented in both type 1 and type 2 DM and has been demonstrated in all age groups. For both type 1 and type 2 DM, its prevalence is estimated to exceed 50%.

Pathophysiology:
In the early morning there is a transient increase in both hepatic glycogenolysis and gluconeogenesis which could raise plasma glucose levels. In healthy individuals plasma insulin levels remain steady through the night, with only a small increase in insulin secretion before dawn. This increase serves to achieve supression of the hepatic glucose production, thereby hyperglycemia is prevented by this physiologic surge of insulin. Hence, the dawn phenomenon does not occur in patients without DM because they can secrete normal amounts of insulin to prevent it. 

There is a similar phenomenon that causes hyperglycemia in the early morning, it is called Somogyi Phenomenon

Somogyi phenomenon is similar to the dawn phenomenon in the sense that both of them cause hyperglycemia in early morning, but with one important difference: Somogyi is preceded by hypoglycemia during the late evening (induced by insulin) which then causes a counterregulatory hormone response that produces hyperglycemia in the early morning. 
The hormones implicated in this phenomenon are glucagon, epinephrine, growth hormone, and cortisol.

This phenomenon is actually less common than the dawn phenomenon. Some researchers even consider it rare.

باختصار:

The dawn phenomenon IS NOT PRECEDED by evening hypoglycemia.
The Somogyi phenomenon IS PRECEDED by evening hypoglycemia.

As for their similarity: they are both "early-morning hyperglycemia."

ماعرف ليش اني لازك بسوالف السكر هالأيام...
بس هو الموضوع متشعب، تفوت بدربونة تطلع من دربونة ثانية

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دتكفرني كمية الأشياء اللي أكتبها هنا، وراها بفترة لما انسئل عنها أنساها...

ليهسة أتذكر بالباطنية مال الرابع بسشنات النيورو، انسألت على الـ UMN lesions وما عرفت الجواب... واني قبلها بسنة بالضبط شارحها هنا :")

المهم...

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Within 10 years of developing overt symptoms of diabetic autonomic neuropathy, 30–50% of patients are dead, many from sudden cardiorespiratory arrest. Patients with postural hypotension (a drop in systolic pressure of 30 mmHg or more on standing from the supine position) have the highest subsequent mortality.