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 thermo-regulation 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).

To sum up, it's generally accepted that 'temperature-senstive neurons" in the Preoptic Area (POA) of the brain are involved in temperature sensing, but it's not known how they work.

In addition, it's also unknown how the POA signals and controls heat generation (or heat loss strategies) in the rest of the body.

[…] the precise nature of the POA projections that play a role in suppressing thermogenesis have not been elucidated.

Source : PNAS February 21, 2017 114 (8) 2042-2047

Also see : Body Temperature Declineplugin-autotooltip__plain plugin-autotooltip_bigBody Temperature Decline

A 2020 study from Stanford University has found that human body temperature - previously standardised at 37°C - has been steadily declining over the last 200 years or so.

Using historical records, the research team found that the current average body temperature is probably now around half a degree lower.

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