supposedly an ice bath can somehow stimulate the vagus nerve
"Along with the radical increase in norepinephrine during cold therapy, short periods of mild hypothermia also activate a release of dopamine in your brain. This, paired with soaring levels of norepinephrine, explain why you can’t help but smile and feel incredible after you’ve plunged into a cold bath. It’s also what makes cold water therapy such a promising treatment pathway for diseases like Alzheimer’s and Parkinson’s"
https://discovermonk.com/blogs/blog/neuroprotec....

Neuroprotection and neuroplasticity benefits of cold water therapy

https://discovermonk.com/blogs/blog/neuroprotection-and-neuroplasticity-benefits-of-cold-water-therapy