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Research indicates that extrinsic motivation will displace a person’s internal motivation. This is called the overjustification effect.

Lack of adequate sleep increases the production of the appetite-increasing hormone ghrelin while decreasing production of the appetite-suppressing hormone leptin

The body creates a state of tension, known as a “drive,” if any of its needs are unmet

According to the principle of homeostasis, the body seeks to maintain a stable internal state, such as constant internal temperature and fluid levels. For example, after completing a marathon, runners drink a large quantity of water to restore homeostasis in their fluid levels.