To ensure that this urge doesn't awaken us in the middle of the night, the kidneys process fluid differently while we sleep so that the urine is concentrate d and therefore of lower volume. That's why morning urine is darker in color than that produced while we're awake. The aroma of, say, eggs sizzling in a frying pan reaches the mucous membranes lining the nose an d stimulates olfactory nerve receptors, which transmit the impulse to the brain, where it is recognized as an odor. In response to odor and even just the thought of food, the brain, working through the endocrine and nervous systems, transmits messages to glands in the mouth and stomach, which increase the flow of digestive juices in anticipation. The sense of hearing also comes into play.
Sight is yet another important sensory response in readying the body for a meal.
Light from an object, such as a glass of juice, strikes the lens of the eye and is directed to the retina. The impulse generated by the retina travels along the optic nerv e to the brain, where it is perceived. If the image shows a favorite food, our appetite center and digestive system will be further stimulated. Touch is also involved in eating. As the food touches nerve endings in the tongue and palate, its texture and temperature further stimulate circulation of blood in the entire digestive system and also help increase the secretion of digestive enzymes.