In between meals, after all your food is gone in the GI tract....

your liver responds to blood sugar levels, and when they drop it then converts amino acids to glucose. This is called gluconeogenesis.

Metformin blocks this process as part of how it works BTW.

So if you do not eat anything after 7pm, you will have a drop in blood sugar
before morning. Diabetics tend to have exaggerated responses to this.

An apple or low carb snack will be SLOWLY digested and the sugars it does contain released slowly so that the hypo dip does not occur.
A cookie/brownie or whatever, will cause a sudden blood sugar rush and then a bigger dip in the night later. The bigger the dip the more the liver responds to
the dip and MORE sugar is made and hence you test higher.
Glucerna bars are designed to be digested slowly, and so the hypo dip for diabetics is blunted and the blood sugar remains more normal.

When you eat that low carb, high fish high protein diet....you are giving your liver amino acids to be converted to sugar. Because this is a slow process the overall effect is to control wide swings up and down.