“The core temperature of a human is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system, and more specifically by the anterior hypothalamic nucleus and the adjacent preoptic area regions of the hypothalamus. As core temperature varies from the set point, endocrine production initiates control mechanisms to increase or decrease energy production/dissipation as needed to return the temperature toward the set point.”
This quote from Wikipedia (also see diagram) mentions the set point (around 37°C for humans). The entire thermoregulation system relies on this point as a reference by which to adjust the body temperature either up or down as required. However, the mechanism by which the set point is 'set' is unknown.
“Temperature detection and regulation is of vital importance to any homeothermic organism. In order to maintain temperature homeostasis it is necessary for the autonomic nervous system to monitor small fluctuations in core body temperature and initiate counter measures to prevent temperature fluctuations beyond a tightly controlled set point. Key brain centers concerned with temperature control are the preoptic area and anterior hypothalamus (PO/AH). These hypothalamic regions harbor neurons that not only detect changes in core body temperature, but are also believed to receive and integrate input from ascending somatosensory pathways carrying information from peripheral temperature sensors.
The molecular machinery underlying central temperature detection by hypothalamic neurons is currently unknown.”
Source: Temperature Detection and Thermoregulation Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC).
Also see : Body Temperature Decline