A 15-hour fast produces physiology similar to 16 hours: insulin tends to decline, fat oxidation contribution rises, and glycogen gradually decreases. For many people, it is easier to implement consistently by finishing dinner earlier and delaying breakfast modestly. Consistency over weeks matters more than a single-hour difference. Some experience reduced morning appetite, while others feel hunger. If dizziness, shakiness, or irritability occurs, soften the window (12–14 hours), raise protein/fiber at the first meal, and optimize hydration/electrolytes. Sustainable structure beats relying on heavy caffeine to suppress hunger.