Internal stimuli are thoughts or physiological sensations that trigger a living being to do something. The internal stimulus of hunger, for instance, triggers normal living things to seek food.
Internal stimuli ensure living things are motivated to perform life-sustaining things, like eat, excrete and reproduce. While many internal stimuli, like hunger or sleepiness, occur spontaneously, most internal stimuli are reactions to external stimuli. For example, a male cat's stimulus to reproduce is triggered when he smells, hears or sees a female cat in heat.
Instincts are intertwined with internal stimuli as well. These complex interactions depend on internal stimuli, external stimuli and a programmed response to work properly. An example would be the instinct of birds to fly south for winter. They are externally stimulated by a drop in temperature and internally stimulated by hunger caused by food scarcity, so they instinctively fly southward where it's warmer and more food is available.
An understanding of how internal stimuli work is also important to marketers and others who depend on the manipulation of human behavior. For example, restaurants have learned to vent food smells outside into sidewalks and streets. The odor becomes an external stimulus that triggers hunger or appetite and may drive customers into the restaurant to soothe that internal stimulus.