Your body makes extra blood sugar (glucose) overnight in order to avoid periods of low blood sugar (hypoglycemia) while you're sleeping and to give you the energy you need to get out of bed. In people without diabetes, insulin — the hormone that regulates blood sugar — also rises to keep blood sugar stable.
Some people's blood sugar is extra-sensitive to caffeine. Losing sleep—even just one night of too little sleep can make your body use insulin less well. Skipping breakfast—going without that morning meal can increase blood sugar after both lunch and dinner.
Fasting hyperglycemia is defined as high blood sugar when you haven't eaten for at least eight hours. The recommended blood glucose range after fasting in a person without diabetes is 70 to 130mg/dL. (The standard for measuring blood glucose is "mg/dL" which means milligrams per deciliter.)
A skipped meal alters the balance between food intake and insulin production, and can cause your blood sugar levels to eventually drop. “For diabetic people dependent on insulin or blood sugar–lowering medication, skipping meals can be more dangerous because it can lead to low blood sugar,” says Pearson.
After a period of fasting, insulin sensitivity increases, and insulin levels decrease. These changes result in improved blood sugar levels both during fasting and shortly after eating.
Fasting can definitely raise blood glucose. This is due to the effect of insulin falling and the rising counter-regulatory hormones including increased sympathetic tone, noradrenaline, cortisol and growth hormone, in addition to glucagon.
Even non-sweet carbohydrates like starchy foods undergo metabolic conversion to simple sugar. Consequently, the blood glucose levels start to increase. In some cases, a low-carb diet can also elevate blood sugar. If you experience this, try including high-fibre in the diet.
Drinking water regularly may rehydrate the blood, lower blood sugar levels, and reduce diabetes risk ( 20 , 21 ). Keep in mind that water and other zero-calorie drinks are best. Avoid sugar-sweetened options, as these can raise blood glucose, drive weight gain, and increase diabetes risk ( 22 , 23 ).
Fast for 12 hours a day
According to some researchers, fasting for 10–16 hours can cause the body to turn its fat stores into energy, which releases ketones into the bloodstream. This should encourage weight loss. This type of intermittent fasting plan may be a good option for beginners.
High levels of fasting blood sugar suggest the body was not able to lower blood sugar levels. This points to insulin resistance, inadequate insulin production, or in some cases, both. Very low blood sugar levels may indicate that diabetes medications are lowering the blood sugar too much.
Common fasting blood tests
a fasting blood glucose test (used to test for diabetes) – you may be asked to fast for 8 to 10 hours before the test. an iron blood test (used to diagnose conditions such as iron deficiency anaemia) – you may be asked to fast for 12 hours before the test.
What is 'fasting' and how does it work? A good rule of thumb for those wanting to fast to improve their metabolic and overall health is to leave at least 12 hours between meals – ideally more – on a regular basis, according to Dr Adam Collins, Principal Teaching Fellow in Nutrition at the University of Surrey.