You don't need a medical degree to know this. I know it from personal experience, and from studying experiments on how some people can become conditioned to surviving cold temperatures which would quickly kill most other people.
I also know it from being an engineer who helped develop vital signs monitors, and from participating in one of the early tests myself, which involved treading water in cold water for several hours.
Scientists use thermal imaging and internal sensors inserted through the nasal passage and esophagus to monitor core body temperatures.
