The hypothalamus is located below the thalamus, just above the brain stem. It is responsible for a variety of the autonomic nervous system. One important function of the hypothalamus is linking the nervous system to the endocrine system, via the pituitary gland.
The hypothalamus responds to many different signals, both internal and external to the body. As examples of internal stimuli, it responds to neural signals from the heart, stomach and reproductive tract. It also responds to hormones carried in the blood, olfactory stimulation and steroid stimulation. External factors affecting the hypothalamus include stress that triggers a variety of responses, and light that controls functions such as sleep.
In response to these internal and external signals, the hypothalamus controls certain metabolic processes and other activities of the autonomic nervous system. It is responsible for increasing and decreasing the heart rate, as well as blood pressure. It controls thirst and hunger, and stimulation of the gastrointestinal (GI) tract. It not only tells you that you're hungry, but also tells you that you're full. Through the endocrine system, the hypothalamus controls emotional behavior such as mood, anger and sex drive.
The hypothalamus controls thermoregulation, or heat regulation, maintaining a normal body temperature. If you get cold, it stimulates heat production and retention to raise the blood temperature. If you get hot, it controls sweating and vasodilation to cool the blood, and thereby cool the body. It also controls the fever response, raising the body temperature to help fight off invading microorganisms. It also regulates circadian and seasonal rhythms, controlling functions such as sleep.