Comment by prostoalex

+1. It's all about the insulin release. After a year or so on a zerocarb diet I'd allow myself an occasional feast of fresh fruit, but in the process my body has become so sensitive to insulin, a fresh spike of it would lead to an immediate loss of energy and sleepiness regardless of the time of day.

I've tracked my weight and did blood tests consistently, and most dramatic weight loss periods coincided with the minimal insulin presence.