Agni is the term Ayurveda uses for digestive fire, and it carries more weight than the English phrase suggests. It refers not only to what happens in the stomach but to the body's whole capacity to transform — food into tissue, experience into understanding. Classical texts return to it constantly, and the reason is a single line from the Ashtanga Hridayam: all disease begins with a weakened digestive fire.
What is described is not one fixed capacity but four states. Sama agni is balanced: hunger arrives on time, digestion is comfortable, the bowel is regular. Vishama agni is irregular, associated with Vata — appetite that comes and goes, bloating, unpredictability. Tikshna agni is sharp, associated with Pitta — intense hunger, irritability when a meal is late, acidity. Manda agni is slow, associated with Kapha — heaviness after eating, appetite that never quite arrives.
Recognising which pattern you tend towards is more useful than any single dietary rule, because the correction differs. Irregular digestion is helped by regularity above all: same meals, same times, warm and cooked. Sharp digestion is helped by not skipping meals and by cooling, less pungent food. Slow digestion is helped by lighter food, longer gaps, and by not eating simply because it is mealtime.
One piece of advice applies across all three. The main meal belongs at midday, when digestive capacity follows the sun and is at its peak. The same food eaten at ten at night is not the same food, because the body it arrives in has a different capacity to handle it.
If digestion has been unreliable for a long stretch, it is worth having it looked at properly rather than adjusting the diet indefinitely on your own. Persistent change in digestion deserves examination.