You know why. I’ve been writing about it for years. Happens also to pasta.

Influence Of Resistant Starch Resulting From The Cooling Of Rice On Postprandial Glycemia In Type 1 Diabetes, Nutrition and Diabetes, 16 April 2022

Each participant of the study consumed two standardized test meals consisting of long-grain white rice. One of the test meals was served immediately after preparation, and another was cooled for 24 h at 4 °C [about 40 degrees F, refrigerator temp] after preparation and reheated before being served.

Conclusions: Consumption of rice subjected to the cooling process results in a lower increase of postprandial blood glucose in subjects with type 1 diabetes.

So, a delayed blood glucose peak, and a lower peak. Why? Resistant starch:

The cooling of rice after cooking causes retrogradation of starch, which becomes a non-absorbable product in the human digestive tract.

Note that the rice was reheated after being cooled. It did not substantially lessen the resistant starch formed during cooling. Also, “multiple heating/cooling cycles of starch products increased the resistant starch content even more.”

These resistant starch calories are, by the way, calories we do not absorb.

Also:

It is believed that the content of resistant starch in the diet has a beneficial effect on reducing the degree of hunger and the desire to eat and increasing the feeling of fullness.*

* Type-4 Resistant Starch in Substitution for Available Carbohydrate Reduces Postprandial Glycemic Response and Hunger in Acute, Randomized, Double-Blind, Controlled Study, Nutrients, February 2018