This section is written by Charles Coughran <ccoughran(AT)ucsd.edu>.

There are three main causes of high morning fasting bg. In decreasing
order of probability they are insufficient insulin, dawn phenomenon, and
Somogyi effect (aka rebound). Insufficient or waning insulin is simple.
If the effective duration of intermediate or long acting insulin ends
sometime during the night, the relative level of circulating insulin
will be too low, and your blood sugars will rise.

Dawn phenomenon refers to increased glucose production and insulin
resistance brought on by the release of counterregulatory hormones in
the early morning hours near waking. It happens in normal people as well
as in diabetics; in nondiabetics it shows up as measurably increased
insulin secretion around dawn. Dawn phenomenon is variable in strength
both within the population and over time in individuals. It can show up
as either high fasting glucose levels or an increased insulin
requirement to cover breakfast compared to equivalent meals at other
times of day.

Somogyi effect refers to a rebound in bg after nocturnal hypoglycemia
which occurs during sleep with the patient not experiencing any
symptoms. The hypoglycemia triggers the release of counterregulatory
hormones. Somogyi effect appears to be less prevalent than previously
thought. While it does occur, some episodes of hyperglycemia following
hypoglycemia are actually waning insulin levels following an insulin
peak with medium acting insulin. This can be difficult to sort out.

The best way to sort it out is to test every couple of hours from
bedtime to morning.

     If your bg rises all, or much of the night, it is a lack of
     circulating insulin.

     If it is stable all night, but rises sharply sometime before you
     wake in the morning, it is dawn phenomenon.

     If your bg declines to the point of a hypoglycemic reaction, it is
     *possibly* Somogyi effect.

You may have to test on several nights to nail the problem. Once you
have figured out the problem you and your doctor can discuss changes in
your insulin regimen to correct it. The answer depends critically on
your particular circumstances.

Mayer Davidson, in _Diabetes Mellitus: Diagnosis and Treatment_ (p 252
in the 3rd edition) says that Somogyi effect rarely causes fasting
hyperglycemia, and cites studies.