Hunger

Hunger is a feeling experienced when the glycogen level of the liver falls below a threshold, usually followed by a desire to eat. The usually unpleasant feeling originates in the hypothalamus and is released through receptors in the liver. Although an average nourished human can survive weeks without food intake, the sensation of hunger typically begins after several hours without eating.

Hunger can also be applied metaphorically to cravings of other sorts, e.g. "hungry for victory."

Hunger pains
When hunger contractions occur in the stomach, the person sometimes experiences mild pain in the pit of the stomach, called hunger pangs. Hunger pangs usually do not begin until 12 to 24 hours after the last ingestion of food, in starvation. A single hunger contraction lasts about 30 seconds, and pangs continue for around 30-45 minutes, then hunger subsides for around 30-150 minutes. Individual contractions are separated at first, but are almost continuous after a time. Emotional states (anger, joy etc.) may inhibit hunger contractions. Levels of hunger are increased by lower blood sugar levels, and are higher in diabetics. They reach their greatest intensity in 3 to 4 days and may weaken in the succeeding days, though hunger never disappears. Hunger contractions are most intense in young, healthy people who have high degrees of gastrointestinal tonus. Periods between contractions increase with old age.

Satiety
Satiety, or the feeling of fullness, is the disappearance of hunger after a meal. It is a process mediated by the ventromedial nucleus in the hypothalamus. It is therefore the "satiety center."

Various hormones, first of all cholecystokinin, have been implicated in conveying the feeling of satiety to the brain. Leptin increases on satiety, while ghrelin increases when the stomach is empty.

Therefore, satiety refers to the psychological feeling of "fullness" or satisfaction rather than to the physical feeling of being engorged, i.e. the feeling of physical fullness after eating a very large meal. Satiety directly influences feelings of appetite that are generated in the limbic system, and hunger that is controlled by neurohormones, especially serotonin in the lateral hypothalamus.lll

Behavioral response
Hunger appears to increase activity and movement in many animals - for example an experiment on spiders showed increased activity and predation in starved spiders, resulting in larger weight gain. This pattern is seen in many animals, including humans while sleeping. It even occurs in rats with their cerebral cortex or stomachs completely removed. Increased activity on hamster wheels occurred when rats were deprived not only of food, but also water or B vitamins such as thiamine This response may increase the animal's chance of finding food, though it has also been speculated the reaction relieves pressure on the home population.