Why do I get cravings at night? by Connie b. Dellobuono
Answer by Connie b. Dellobuono:
There is no single explanation for food cravings, and explanations range from low serotonin levels affecting the brain centers for appetite to production of endorphins as a result of consuming fats and carbohydrates. Foods with high levels of sugar glucose, such as chocolate, are more frequently craved than foods with lower sugar glucose, such as broccoli, because when glucose interacts with the opioid system in the brain an addictive  triggering effect occurs. The consumer of the glucose feels the urge to consume more glucose, much like an alcoholic, because the brain has become conditioned to release "happy hormones" every time glucose is present. There is evidence that addiction and food craving activate some of the same brain areas. Specifically, when smokers look at pictures of people smoking it activates the same areas of the brain as when obese people look at pictures of food.
The cravings for certain types of food are linked to their ingredients. Chocolate for example, contains the neurotransmitter phenylethylamine, which is important for the regulation of the body’s release of endorphins. Endorphins are released following a stressor and result in a sense of relaxation. Exercise and sleep are two alternative ways to help facilitate the release of endorphins.
Chocolate also contains large quantities of iron, which can be depleted during the menstrual cycle. Another common craving is salt. Craving salt may be partly due to being dehydrated. When dehydrated the body loses water, electrolytes, and salt, and by ingesting salt, water retention can be increased. Craving salt can also be a sign of diabetes, heart disease, and sickle cell anemia.
Carbohydrates, or particularly sugars, are yet another common craving. These cravings occur often in the middle of the afternoon when energy is at its lowest.
The craving of non-food items as food is called pica.
The pituitary gland in the brain is responsible for food cravings, sleep, sex hormones and stress.