Hunger is a feeling experienced usually followed by a desire to eat. The often unpleasant feeling originates in the hypothalamus and is released through receptors in the liver. Although an average nourished individual can survive weeks without food intake, the sensation of hunger typically begins after a couple of hours without eating and is generally considered quite uncomfortable. The sensation of hunger can often be alleviated and even mitigated entirely with the consumption of food.
Kamala Markandaya, the Indian writer, evokes the horror and pain of hunger in personal terms:
For hunger is a curious thing; at first it is with you all the time, waking and sleeping and in your dreams, and your belly cries out insistently, and there is a gnawing and a pain as if your very vitals were being devoured, and you must stop it at any cost and you buy a moment’s respite even while you know and fear the sequel. Then the pain is no longer sharp but dull, and this too is with you always, so that you think of food many times a day and each time a terrible sickness assails you, and because you know this you try to avoid the thought, but you cannot, it is with you. Then that too is gone, all pain, all desire, only a great emptiness is left , like the sky like a well in drought, and it is now that the strength drains from your limbs and you try to rise and find you cannot, or to swallow water and your throat is powerless, and both the swallow and the effort of retaining the liquid tax you to the uttermost.
— Kamala Markandaya
When hunger contractions occur in the stomach, these are 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 certain amount of 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.
The fluctuation of leptin and ghrelin hormone levels results in the motivation of an organism to consume food. When an organism eats, adipocytes trigger the release of leptin into the body. Increasing levels of leptin results in a reduction of one's motivation to eat. After hours of non-consumption, leptin levels drop significantly. These low levels of leptin cause the release of secondary hormone, ghrelin, which in turn reinitiates the feeling of hunger.
Some studies have suggested that an increased production of grehlin may enhance desire towards perceptive food cues, while an increase in stress may also influence the hormone's production. These findings support why hunger can prevail under stressful situations.
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.