I did alot of research into postprandial somnolence but I wasnt able to find a consistent cause for it. Theres lots of ideas floating around, including more tryptophan getting into the brain and being converted to serotonin, but studies are inconsistent.

The only thing I have seen to reliably produce postprandial somnolence is a high-calorie intake at a single sitting. But even this is not 100% consistent.

Currently, no-one knows the cause of postprandial somnolence