According to the U.S. National Institutes of Health (NIH), a blood glucose level below 70 mg/dL (3.9 mmol/L) at the time of symptoms followed by relief after eating confirms a diagnosis for reactive hypoglycemia. Signs and symptoms
Symptoms vary according to individuals hydration level and sensitivity to the rate and/or magnitude of decline of their blood glucose concentration.A crash is usually felt within four hours of heavy carbohydrate consumption. Along with the symptoms of hypoglycemia, symptoms of reactive hypoglycemia include:

The majority of these symptoms, often correlated with feelings of hunger, mimic the effect of inadequate sugar intake as the biology of a crash is similar in itself to the bodys response to low blood sugar levels following periods of glucose deficiency. Causes
The NIH states: "The causes of most cases of reactive hypoglycemia are still open to debate. Some researchers suggest that certain people may be more sensitive to the body’s normal release of the hormone epinephrine, which causes many of the symptoms of hypoglycemia. Others believe deficiencies in glucagon secretion might lead to reactive hypoglycemia.Several other hormones are responsible for modulating the bodys response to insulin, including cortisol, growth hormone and sex hormones. Untreated or under-treated hormonal disorders such as adrenal insufficiency (see also Addisons disease) or growth hormone deficiency can therefore sometimes cause insulin hypersensitivity, and reactive hypoglycemia. Stomach bypass surgery or hereditary fructose intolerance are believed to be causes, albeit uncommon, of reactive hypoglycemia.
Another cause might be hysteresis effect of insulin action, i.e., the effect of insulin is still prominent even if both plasma glucose and insulin levels were already low, causing a plasma glucose level eventually much lower than the baseline level.Sugar crashes are not to be confused with the after-effects of consuming large amounts of protein, which produces fatigue akin to a sugar crash, but are instead the result of the body prioritising the digestion of ingested food.The prevalence of this condition is difficult to ascertain because a number of stricter or looser definitions have been used. It is recommended that the term reactive hypoglycemia be reserved for the pattern of postprandial hypoglycemia which meets the Whipple criteria (symptoms correspond to measurably low glucose and are relieved by raising the glucose), and that the term idiopathic postprandial syndrome be used for similar patterns of symptoms where abnormally low glucose levels at the time of symptoms cannot be documented. To assist in diagnosis, a doctor may order an HbA1c test, which measures the blood sugar average over the two or three months before the test. The more specific 6-hour glucose tolerance test can be used to chart changes in the patients blood sugar levels before ingestion of a special glucose drink and at regular intervals during the six hours following to see if an unusual rise or drop in blood glucose levels occurs.