why am i not hungry after not eating all day

The reason for not feeling hungry after not eating all day is mainly due to the body's stress response and hormonal regulation. When not eating for a long time, the body increases stress hormones like adrenaline and cortisol, which suppress hunger and mobilize energy from stored fat and glycogen. This is part of the "fight-or-flight" response, helping the body to function without immediate food intake. Additionally, the body regulates blood sugar through increased gluconeogenesis (creation of glucose from non-carbohydrate sources) which keeps energy levels stable, reducing the sensation of hunger temporarily. Other factors that can suppress hunger include anxiety, stress, illness, or even eating a large meal late at night. Hunger is normally regulated by signals from the hypothalamus in the brain, along with hormones such as ghrelin, but these signals can be overridden by stress hormones in some situations. This is why sometimes people who haven't eaten for a day might still not feel hungry until those stress hormones decrease or energy stores become depleted enough to trigger hunger again.