The hypothalamus is the part of the brain that controls the automatic nervous system. This part of the nervous system is usually unconscious activity and has large influence over internal organ functions. Specific functions regulated by the ANS include heart and breathing rates, urination and defecation, body temperature, and sexual response.
It further breaks down into the sympathetic and parasympathetic nervous systems. The first regulates the so called fight-or-flight instinct in regards to adrenaline. The second is more of an influence over to when to rest, eat and sleep.