Temperature regulation
Your body is made to regulate your temperature and to keep all other systems in homeostasis.
Your temperature is regulated from the centre of your brain.
Research at AMC Amsterdam Medical Center showed that poeople ending there showers cold for 30 days results in 29% less sick days.
https://www.parool.nl/nieuws/elke-dag-een-koude-douche-leidt-tot-minder-ziekteverzuim~bf3a97c7/?referrer=https%3A%2F%2Fwww.google.com%2F