Low blood sugar. It can be an actual clinical condition (rather rare), but the term is usually applied to LABILE blood sugar, where the highs are socially acceptable, if zappy, but the lows cause headaches, depression...and sugar cravings...which only kick the sugars UP...which forces the sugars DOWN...etc. This is a subclinical condition that usually goes nowhere, at least clinically, but can drive you (or your companion) crazy. Some normal and healthy food have a rapid blood presence and can leave you hanging; fruits, potatoes and carrot juice are LOUSY...legumes, particularly beans, supply slow and extended release of calories...mostly because of high levels of soluble fiber...and laborious digestion. Even adding such dietarily useless items like Psyllium Seed and Chia Seed can do wonders to slow down sugar spiking.